I don't care if you believe me; it's completely reproducible so I KNOW that's what worked for me, even if it goes against everything I know about software development (which is a considerable amount). 🙂
Changing sound settings didn't help at all. I had it set to Stereo, 16bit, 44.1khz, all effects disabled, spatial sound disabled...no dice. Even connected some audio devices that only support stereo (and disabled/uninstalled all others). Didn't help either.
I wish we had access to FA's source code; I'm fairly sure we could fix a lot of things.
Weirdly enough, while others found the same solution (https://steamcommunity.com/app/9420/discussions/0/1620600279663590987/), for some people the problem was fixed by moving Forged Alliance TO the boot volume (https://steamcommunity.com/app/9420/discussions/0/1473096694446040381/).