I am running UT99 with the Kentie DX10 mod, and I added in map pack 4, which for some reason wasnt included in the gog version, the pack that added Ratchet and a couple other good CTF and DM/TDM maps.

The music is fine, however there is a ton of distotion and popping from gun fire and speech, been playing this off and on for years, haven't had this happen in forever, since long before it was a gog title.
Weirdest part, I played a round of it on Friday, and it was fine, only things I set in preferences was an FPS cap to 144 which is my monitor refresh. Didn't touch the sound. Since then I tried 3D sound as I do have a SB card, but didnt help.

Any suggestions?

Looks like I needed to mess with an ini file.

UnrealTournament.ini in System folder
[Galaxy.GalaxyAudioSubsystem]
UseDirectSound=True

And set this parameter to "False"

Now I get a less annoying error, the audio channels seem to cutr in and out, where they didnt before, so that is odd but definitely more usable now.

If anyone has any insight on that, it would be appreciated.

You could try running the game in compatibility mode for Windows XP.