[SDPhantom]

The lines responsible for playing the target select and lost sounds are here
https://github.com/Gethe/wow-ui-sour....lua#L172-L178
https://github.com/Gethe/wow-ui-sour...Frame.lua#L258

There's also an API function for muting specific sounds by either path or FileID.
MuteSoundFile()

Taking the SoundKitIDs and looking up their FileIDs, these calls should do the trick.

MuteSoundFile(567453);
MuteSoundFile(567520);