To all those having a problem replacing the firing sound, I have found how to get it working!
While it appears that fl_pistol.WAV is the only sound file for guns, there are actually 5 more that need to be replaced to get it to work.
So, if you want to change the firing sound, replace these files:
fl_pistol.WAV
poroh_babah1.ogg
poroh_babah2.ogg
poroh_babah3.ogg
poroh_babah4.ogg
poroh_babah5.ogg
I replaced them all with a musket sound from a Napoleon Total War mod, and it now sounds very awesome!