EDIT:
Currently the game has issues with various sound cards and speaker configurations as follows:
2.1
4.1
5.1
6.1
7.1

WORKAROUND: at present the workaround is to set speaker configuration to Stereo (2.0) or Quadraphonic (4.0) until the game is fixed.

NOTE: This workaround only appears to work for some users and hardware configurations.
It did not work for me when output from either the Onboard RealTek Audio or the NVIDIA High Definition Audio via HDMI out from GTX 760

There is a reply with links to many more folks having this problem and documenting it out on different sound cards and consoles: https://www.gog.com/forum/the_witcher_3_wild_hunt/only_stereo_and_very_low_volume/post5

NOTE: This issue appears to be affecting the Xbox One and PS4 versions of the game as well as the PC version! Seriously, how does that get missed, they are using fixed hardware and fixed audio routines on the consoles, can't blame different OSes, driver configs, sound cards, etc.

EDIT: NOTE: There appears 2 distinct issues at play here. I detail them both out below in this reply to the thread:
https://www.gog.com/forum/the_witcher_3_wild_hunt/only_stereo_and_very_low_volume/post33

There may be other issues at play as well, but both of those issues are easily confirmed/proven and tested. As such they are listed as confirmed issues here which CDPR need to address and fix.

-----------------------------
Original post follows:
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Only Stereo and very low volume?

Windows itself knows I have a 7.1 Surround sound configuration. My other games seem to play out of all speakers correctly.

TW3, only the front left & right and at very low audio levels even when the game is set to 100% (at first I was hoping it was just the introductory cut-scenes).

This is unplayable, my email client message arrived sound is blaring when it plays while I have the receiver volume cranked enough to even hear the dialog during the opening cut scenes.

Version in game says 1.02 on the Main Menu

This is totally unplayable like this.

I normally have my sound on the receiver at an output of 10-12.
To even be able to hear the opening cut-scenes I've had to crank the volume to 44 on the receiver.

Having an email come in nearly knocked me out of my seat it was so loud.

I refuse to consider the only workaround to go into the windows control panel, and disable all windows sounds, and kill every other running application that might produce sound just so I don't risk blowing my speakers.

Unacceptable.

same problem here.
ingame volume is far too low...

Using watercooling by any chance? The dim sound and cracking sounds like what happened to mine after my WC leaked all over my sound card :D

I test it when I get in game (on a Asus Xonar STX)

Nope, and as I said, audio is fine for everything else. My new email message arrived notice nearly knocked me out of my seat I had to turn the volume up so loud to hear the game (which of course made it super loud for everything else).

I'm going to use this reply to link in more of these reports. Based on the current list it would appear a wide range of sound cards and hardware have been confirmed to have been impacted in this issue including BOTH consoles (PS4 and Xbox One).
More and more looking like the problem is anyone with a configuration of 2.1, 4.1, 5.1, 6.1 or 7.1 are impacted.

Steam user with the same problem it would appear:
https://steamcommunity.com/app/292030/discussions/0/617335934148118510/
This thread has more people adding to it as this was typed.

And another user who is obviously having the same problem based on their suggesting using driver settings to boost windows audio globally:
https://steamcommunity.com/app/292030/discussions/0/617335934148107798/

And this post would seem to indicate only STEREO is supported as folks are switching to Stereo to solve the sound issues on 2.1 and higher configurations:
https://steamcommunity.com/app/292030/discussions/0/617335934148137250/

And more reports:
http://steamcommunity.com/app/292030/discussions/0/617335934148195755/
http://steamcommunity.com/app/292030/discussions/0/617335934148174633/
http://steamcommunity.com/app/292030/discussions/0/617335934148129705/
http://steamcommunity.com/app/292030/discussions/0/617335934150344348/
https://www.gog.com/forum/the_witcher_3_wild_hunt/surround_sound_not_working_51
https://www.gog.com/forum/the_witcher_3_wild_hunt/voice_over_audio_dosent_play
http://forums.cdprojektred.com/threads/36668-Very-low-sound-volume
http://forums.cdprojektred.com/threads/36819-no-voices-on-PC
http://forums.cdprojektred.com/threads/36827-No-audio-during-cutscenes
http://forums.cdprojektred.com/threads/37150-5-1-audio-broken-XBOX-One-probably-other-platforms-also
http://forums.cdprojektred.com/threads/36843-gerald-voice-either-mute-or-in-very-high-sorround-barely-hearable
http://forums.cdprojektred.com/threads/36851-Problem-with-sound-for-dialogues
http://forums.cdprojektred.com/threads/36856-Problem-with-the-Voices-of-the-Npc-s
http://forums.cdprojektred.com/threads/36940-Slow-volume
http://forums.cdprojektred.com/threads/36597-Missing-sound
http://forums.cdprojektred.com/threads/37474-PC-Surround-Sound-Not-Working-over-SPDIF

And here is one confirming this issue exists on PC & Xbox One! It is broken even on CONSOLES!
http://forums.cdprojektred.com/threads/36947-TW3-doesnt-work-with-surround-sount-for-me-why

And another that includes reports from PS4 users:
http://forums.cdprojektred.com/threads/36529-PS4-Audio-problem-on-all-character-voice

How many examples are needed given it is so many different people, different systems, and includes both the Xbox One and the PS4?

Going to have to shelf this game for now as it appears this audio issue impacts a majority of 2.1, 4.1, 5.1, 6.1 & 7.1 configurations from a couple different card manufacturers with the only WORKAROUND being to limit sound to Stereo by switching to either Stereo or Quadraphonic (4.0) speaker configurations.

This is just bad. Was this missed, and if so how given that testing on more than Stero should have been a part of any decent test plan?
If it was not missed and was chosen to be released like this, then the question would be why considering this is unacceptable.

I too have an issue with low volume. Had to increase my volume by 40 for it

Please update as soon as a fix is released.
This is a critical issue and I hope it won't take long before it's taken care of.

I have Corsair 2100 wireless headset, with the drivers installed and set as 2.0 instead of 7.1, and a lot of games are like this for me.
Ingame is so low, I have to turn the volume up in games, and when I get out of a game, it is SO loud.

Are all of you on windows 8.1 ?
I have had this issue since installing 8.1
Windows 7 didnt have a single issue with sound, and I had a 5.1 setup AND the corsair 2100 wireless, and every game just worked fine.

i use B&W MM-1 speaker (just stereo) on Windows 8.1 Pro.
I am already running in 2.0 Stereo configuration so i can not switch from x.1 to stereo only.

Any fix would bne highly appreciated.

In searching around for a solution to the main problem I've heard of another user with this issue even though they had speakers set to Stereo already. To fix it they had to go to their speaker configuration, switch to another setting, apply/ok, then switch back to 2.0. Might want to give that a shot and see if it works for you.

My audio hardware is configured to Stereo only with 2 speakers connected and I also experience this problem where dialogue goes to about 1/2 to 1/3 volume on cutscenes occasionally. It does not happen every single time, just sometimes. All other audio in the game is perfect including regular dialogue. My system is capable of quadrophonic or surround but that is not enabled.

A particular cut scene may go quiet or it might play at the proper volume - it seems random.

Unfortunately not a whole lot else we can do but gather reports till CDPR fix the issue. To me I find the game unplayable with these kinds of audio issues.

When it happened (two or three times now) I just turned up my speakers via volume knob temporarily and worked around it. But I just got finished a few hour session a while ago where it never happened at all so it's still playable to me, but that doesn't mean I wont encounter something that ends up inaudible either. If that happens and a reload doesn't solve the issue then I'll probably end up having to wait for a patch too. I've got TW1 and TW2 installed too which I haven't went through completely to fall back on though too. :)