This has happened a lot in my whole playthrough of 60 hours so far. Now I'm in the quest "The Battle of Kaer Morhen and the fighting in the cutscenes has no sound fx making it super anti-climactic and very awkward to watch. Please help!

I'm having the same issue. No sounds in some (not all) cutscenes, or just partial sounds like background explosions or FX and no voices.

I've already tried checking my sound settings in Win 10 are set to stereo and not 5.1, and made sure audio quality is default (16bit 48000hz)

I have this problem as well; windows 10.

Very annoying...

Some things I noticed:
- It's not in all cutscenes: The Undvik giant fight plays all sounds, except in the last cutscene, there is no sound.
- It's there on both integrated sound and my sound blaster omni
- Setting sound to stereo, 2.1 or surround doesn't do anything to solve the issue
- Sound worked correctly before the Blood & Wine expansion and/or 1.21 patch, but I didn't test 1.12 to be honest.

Maybe it's as simple as the references being renamed.

List of cutscenes where sounds are missing (please add):
- Final cutscene of the Undvik Giant fight
- Battle of Kaer Morhen cutscenes

Other thread reporting the same problems:
http://forums.cdprojektred.com/threads/80911-Sound-is-cutting-out-After-patch-1-21-installed-Did-the-fix-and-that-did-not-help?p=2408591#post2408591
http://forums.cdprojektred.com/threads/81261-BUG-1-20-Random-sound-and-dialog-missing

No, unfortunately.
CDproject is investigating the issue, I (and many others) made a ticket directly at cdproject red.

What's clear, is that it's not a user issue or an incident, as the bug is reproducable by many users on wildly different configurations. We'll have to wait until cdproject fixes it.

Having this too, super annoying. Along with the giant cut scene I am also getting it on the fight of Skellig mission too.

As far as i know this sound problem was there prior to 1.21 .

Could be, but I noticed it because I was replaying the game, which I finished first pre-heart of stone (can't remember which patch exactly)

2 playthroughs up to 1,12
1.21 was the first i heard or "saw" mention of this issue.
It's a 1,21 introduced bug

Alright. That still doesn't exclude my claim. I know i noticed this bug before 1.21 personally. Only presumed it's local problem on my end since nobody else mentioned it.

If I may add my grain of salt, had the same issue, with cutscenes eventually being dark screens with subtitles.
I gave up on my save and downloaded a save file from Nexus.

The save from Nexus didn't show the symptoms. I'm now at the last DLC area with no troubles anymore.
Seems like a problem of save corruption somehow.

(No mods were installed back then, was vanilla.)

Give it a try. Back up your savegame folder somewhere, and try another clean save.
There's a bunch of save files on nexus that "checkpoint" through the main story line, so you should be able to fall back not too far away from where you are.
That's worth a shot I guess.