Is there any way to change the main menu music in Portal 2?

If you can find the file for the music than yes, I have done this before with the game Counter Strike Nexon Zombies.

If you find the games directory, im imagining you have a steam installation so it will be something like, Steam/steamapps/common/Portal 2

If you can search through the directory and find the file for the main menu music, take the file cut it and paste in a SAFE location, it is very important you do not delete this file, also open something like a notepad document quickly and write down the name of this file.

Now take the song you want to replace it with, make sure it is the same format (http://online-audio-converter.com/ incase you have to conver it)

Then move this file to the same location that you found the main menu music in the portal 2 directory and rename the song to the EXACT SAME name as the portal 2 main menu music, so if the original music is called mainmenu.wav your song must be called mainmenu.wav and also be a wav file.

Then run the game and if all has gone well than it should work, if not than I am sorry.