As most people probably know, there's no main menu playlist file, but you can manually rename the files so get whichever song you want to play at startup.
At the time of posting, the song which plays at startup is "Hollywood Principle feat. Dr. Awkward - Solar Eclipses", which is 978255848.wem So you can rename 978255848.wem to whatever you like, then rename whichever track you want to play at startup as "978255848.wem" Unfortunately, as of patch v1.29 on 2017-02-21, the main track is now "Firework (Melad Remix)", which is a new track inside MX_MainMenu.bnk and thus cannot be changed using the above method. Apparently, Psyonix just hates their fans.
As of Patch 1.31 on 2017-03-22, all main menu is now in .wem files, but replacing the startup …Read more >