How to enable it?

Try a solution here: [url=https://pcgamingwiki.com/wiki/Glossary:Surround_sound]https://pcgamingwiki.com/wiki/Glossary:Surround_sound[/url]

I don’t have the game yet, so I can’t test it. Is there an option in settings for 3D sound that is grayed out?

Do you have a soundcard that supports Creative ALchemy or the like? In my case I made a new entry in ALchemy, pointed it to the game's installation folder ('D:\Program Files (x86)\GOG Games\Infernal Machine' for me), checked 'install into sub-folder' and pointed that towards the 'Resource' folder inside the installation folder. After that I could enable 3D sound from the launcher.

I think that games like this that rely on the old DirectSound3D / EAX implementation will not work without some kind of intermediate software like ALchemy.

*edit: also had to set duration from 25 to 20 to fix a bit of stuttering in the intro cinematic, so you might want to play with that value if you have the same issue.

If you have a non-Creative card so you can't use Creative's software, you can try IndirectSound - it's an API wrapper that lets old DirectSound 3D stuff work in Vista/7/8/10.

It's right over here.

I only have two speakers now, so I haven't tried it.

If it is a Creative Labs card use this:

https://www.gog.com/forum/general/creative_alchemy

You can also download 3d back from RealTek and try that. Use the same settings as in the Creative Labs link.

I WAS able to enable 3D sound on my 7.1 system with indirectsound using a Asus Phoebus solo soundcard. However, so far I only heared sound from the 2 front speakers. Then again I didn't play for an extensive time to find a place where I could be sure the sound was coming from the back.