Ahhhhhhhhhhh, DOOMDATA, that was it! Thank you for that! I was mounting my"main drive" in DOSBox as "D:" and not "C:" (I think that was for a reason at some point in the past), so there was no C: to write to for that directory. Sure enough it was there on the C: root of my actual machine though, since Doom had worked correctly running it straight through XP.
I moved DOOMDATA over to my main DOSBox file directory, and went ahead and mounted it as C: inside DOSBox instead of D:, and wah-lah the sound appeared!
Thanks for the help, everyone!!
(As an aside, I don't know if there's a parameter to pass to the main Doom executable to look for default.cfg elsewhere, I'll check, but not a big deal really as the solution above is just fine. Thanks again!)