No sound: Options disables it.
 

The options box on the login screen will automatically uncheck sound every time I close it and go back to check.

Volume sliders ingame are up.
Client sound is set to TRUE
Everquest is visible in the volume mixer but shows no activity

Try renaming your eqclient.ini to like eqclient.old.ini to generate a new one.

Alright here's the thing. The sound works fine when I make an entirely new user account and launch P99 through there without making any other changes.

Any ideas why?

A new windows user account?

If so, it may just be muted in the mixer, or trying to use a disabled/unusable device for output.