There is a bug that allows you to play the menu music while you are ingame and even loop it while the first one is still playing with using the mute shortcut. Here is how to recreate the bug:
1. Go on any server you like to
2. Press Ctrl + S to mute the ingame sound
3. Go into settings and turn the audio back on ( with background music playing )
4. Now the menu music should play while being ingame.
5. Repeat step 2. and the music should loop itself