Looping Menu Music
Before we begin, you will need a sound editor that can make loop points (Audacity can't), I recommend "Wavosaur" (which is free), and the screenshots in this tutorial will be of it.
First, open the sound file that you want to have looped. This can be a MP3, WAV, OGG, doesn't matter.
Then, select which part of the sound you want to be looped and click the red "L" button in the top row.
(You can also press Ctrl+A to select the whole track, then the "L" button to loop the entire track)
Then, save it in "yoursourcemod/sound/ui". You have to save it as a WAV.
Next, open your "yoursourcemod/cfg/valve.rc" file, and add the following:
Save and close it.
That is it! Now you should have a looping main menu track!
Keep in mind that WAV files are uncompressed, and hence can take up quite a lot of space, so try to keep the loop short.