I believe it was in 8.0 that the option for custom sounds was removed - as far as I heard, it was removed by accident to fix some other issue.
Perhaps this could make a return in the future? Either in 9.2.5 or DF ?? For clarification, you used to be able to play custom .mp3 files in a sound folder of WoW, in which you could change music and other sounds. I personally used it to change the gun sounds back to their original sounds, and the log in music to a mix of all expansions once the current expansion’s theme got repetitive.