1.Open the weapon's properties and click on the art and sound tab. Click the button underneath where it says 'Idle Sound'. Scroll down in the list that pops up, and look for sounds with the prefix
MUS.
2. If you want to put a song that is not already in the game as the idle sound, first find your data folder and locate the directory named 'Sound'. This is the folder you need to put your song in. You might want to put the file in the subdirectory named 'songs'.
NOTE: If the folders aren't already there, create them yourself.
3. Re-Open the G.E.C.K. and follow the directions in 1.
If you can't find the song in the G.E.C.K, make sure the file is a WAV or MP3. If it isn't you can convert the file using a free program called http://audacity.sourceforge.net/
Keep in mind that if you don't own the copyright to the song, then you can use it for personal use ONLY. Distributing the file with copyrighted music is illegal.