OK this is how to play music in-game. This will only work with Source games and it will only work on multiplayer.
What you need:
dbPoweramp
HLDJ
GoldWave
Source Games
A torrent program
All links will be at the bottom.

1.Install HLDJ and it will be on your desktop.
2.Move the HLDJ folder to Program Files.
3.Make a shortcut for the HLDJ program.
4.Open HLDJ and run through the setup.
5.It say add games click yes.
6.Go to your game directory
Ex. Program Files/Steam/Steamapps/STEAMID/garrysmod/garrysmod
7. Game Engine will be Source
8.Play key will be whatever you want.
9. Make an HLDJ Music folder wherever you want.
10. In audio directory, choose your HLDJ Music Folder.
11.Click the check mark.
12.It will add all you steam games but you are not ready to add music
13. If u didn't already download the programs.
14. Open up dbPoweramp.
15. Select the music file you want.
16.Make the file 16 bit, 1 mono, and 11khz
17. For the output folder pick HLDJ Music
18.Click convert.
19.Open up GoldWave
20.Open the music file in HLDJ Music.
21.Click Effect, Filter ,then Equalizer.
22.In the drop box click reduce bass
23.Click Save.
24. Open up HLDJ click the game you are about to play then press the play button.
25. Open up your game.
26.Play multiplayer
27.In console type "la" then select the number of the song you want to play
28. Press your play key.
29.There you go

Hope you like this tutorial, i worked on it for an hour so you better like it
Driftkiller asked for this. I hope he is happy.

Links:
http://btjunkie.org/torrent/dBpowerAMP-Music-Converter-13-4/38362af27f5f31068ff5733ecda1a40a87417e71b3e1
http://btjunkie.org/torrent/GoldWave-v5-55-Incl-Keygen/4358a0e9cb59886559214420696a7e391da4a6c7c44b
http://www.hldj.org/downloads.html

Hope this will make me respected
http://forum.randomgs.co.cc/index.php/topic,2678.0.html

u bastard, now they fuckin start playin stuff :/
no moar my songs :/

yayz now i can mic spam

lol good job, this will come in handy lol

Nice guide =D
But I use HLSS, more comfortable for me =D

Agreed with HLSS.
Nice guide, but WHY?! I already saw too much people mic spamming since this guide came ;_;

now there are more... :/

sorry if this made you angry. this was a request. BLAME IT ON DRIFTKILLER

No, i was confused. The only problem was the music files, it wasnt on 11025 power, or mono, and shizz. but i figured it out, but this is a great topic, dont blame on me D: