Placing the files in the correct dir.
HL or any other mods take the mp3 starting from the root dir (SteamSteamApps'email-dir'gamevalve) you can place your mp3's there or create a folder.
As an example i've created a folder called 'mp3' in the "SteamSteamApps'email-dir'counter-strikevalve" so I have "SteamSteamApps'email-dir'counter-strikevalvemp3" now.
Now i have add an mp3 called 'my.buddy.mp3' by G-Unit into that dir.


Using the commands to play the MP3.
To play the MP3 you need to type the command 'mp3 play filename' / 'mp3 play dirfile' in your console, remember dont use the file extension.
So in my case it will be 'mp3 play mp3my.buddy', once you press enter it will start playing the song.
To stop playing a mp3 file, use the command 'mp3 stop' (d0h). An aditional command to set the volume of the mp3 not using the options tab is by using the command 'mp3volume 0.8' avail from 0.0 mute to 1.0 full volume.

NOTE If there are space in the filename you need to use mp3 play "filename"
NOTE If for some reason it aint working, try to backup/remove your config and open the game again. If it still isn't working download this file (93kb), its the winamp lama mp3 that worked here, so you can test if its the mp3 or your settings.
It looks like MP3's encoded with WinLAME or Frauenhofer Codec work fine, dont have any further information on that.
Also set developer to 1 using 'developer 1' in the console, that will echo if its playing or not, if you have develop 0 it will start playing without echo it to the console, so if the file isn't correct you wont know it and it just wont start playing.