Yes, you're absolutely right. If it was a physical problem, it would occur with or without sound and obviously not just through my headphones... This must have something to do with the reading/sampling/conversion of audio files somehow.
I don't think anything is broken with my retail FS2, because if I launch the vanilla exe, everything is fine. Interestingly, however, if I launch Retail FS2 with Knossos, I get the same thing. And since I also get the noise if I install FSO the old fashioned way, this must be because of some file added by FSO but that is not related to at least the graphical improvements. Everything seems to point to some incompatibility with OpenAL, even though the debugger says it's loading fine.
That line seems correct to me though - OpenAL soft is used on my audio card which uses Realtek. I can't change the playback device (only this one option), but I'll try force updating the drivers (all mighty Windows had told me they were already up to date, but who knows...)