I assume by fast-forwarding you mean ALT-F12?

Anything's possible if you believe. Or at least if you're willing to tear apart the source code.

It might just be easiest to find a Windows program (AutoHotKey, maybe?) that will let you selectively mute the sound when you press a key.

Yes, that's what I would mean.
I would rather not "believe" but go through the facts, still the source code is like chemistry compared to the alchemy that is closed source software 🤣
Anyway, thanks for the suggestion, I'll try the latter since I can fiddle with C++ code but I can't add or substract stuff, as I don't have enough knowledge, assuming that I have to add code for that specific behaviour to happen

With Autohotkey it should be fairly easy, just make sure the muting is only done while pressing the keys AND DOSBox is active..