Garry's Mod on Linux, custom sounds

[Eptarch]

I play Garry's Mod on Linux distribution (Gnome-Ubuntu 16.04 LTS) via Steam, and I can't play mp3 files through the in-game console (for instance play "filepath/file.mp3")

The error I get in console:

Create Stream Failed error 44
Failed to load sound "doublekill.mp3", file probably missing from disk/repository

The mp3 files I tried to play are in the folder /downloads/sound/ and they are recognized by the game, but they just don't play for some reason.
I googled a lot about the issue, but due to low amount of linux players there isn't a lot of info.

Please, can you help me figure out the issue?

PS I have mp3 codecs installed, and I can play any mp3 through vlc/mplayer/audacious. Maybe there is some mysterious codec I have missed, if so, I'd like to know what is it? mpeg123 is installed, ubuntu-restricted-extras is installed.