Instructions
Visit Shacknews.com and download the archive containing HLSS. Double-click on the archive once your browser has downloaded it and select "Extract."
Click on "Start" and select "Computer" to launch Windows Explorer. Navigate to the folder that contains your Garry's Mod installation. Typically, Garry's Mod is installed in "C:\Program Files\Steam\SteamApps\accountname\garrysmod\garrysmod\," where "accountname" is the name of your Steam account.
Double-click on the "cfg" folder and right-click on the "autoexec.cfg" file. Select "Open with" and click on "Notepad."
Copy-and-paste the following lines into the file:
Click on "File" and select "Save."
Launch the HLSS.exe file that is in the folder you previously extracted. Click on the "Options" button and select "Auto-find" in the Options window. Click on "OK." Keep HLSS.exe open and launch Garry's Mod. You can now play music while running the game.
alias hlss-START "voice_inputfromfile 1; voice_loopback 1; +voicerecord; alias ToggleWAV hlss-STOP"
alias hlss-STOP "voice_inputfromfile 0; voice_loopback 0; -voicerecord; alias ToggleWAV hlss-START"
alias ToggleWAV "hlss-START"
voice_fadeouttime 0