Ok, as far as I know Garry removed MP3 SUPPORT from GMOD, so If you have any addons working with this kind of files, you'll have to convert it to .wav format, cause thats the only one I know is working (OGG seems to be unsupported as for now).
Recheck your addons ppl

Hello ppl, I've released a meteor SWEP which is a port from one of the toybox packs i had lying around on my disk.
I will be currently working on this swep's icon and thet kind of stuff.
Any piece of feedback will be helpful :D

http://steamproxy.net/sharedfiles/filedetails/?id=146820622