No, not really.
Odd that they hear some but not all.
There is a server variable that limits the size of files that can be sent to users. Perhaps your is set high enough for the wavs, but not high enough to send the mp3?
Also, test it yourself.
Your custom directory is named gmod, yes? /gmod9/sound/gmod?
Remove your custom sounds from your game directory /blah/gmod9/sound/gmod/....
Leave the sounds in your Source dedicated server/gmod9/sound directory
Make sure you don't have the same sounds in your gmod9/mods/~modcache/sound/gmod directory
Then, connect to your server, see if all download, and see if they can be played.
Then, search your game directory for the downloaded sounds.
Other than that, I dont have an idea what to try