The mope.io music are looping theme songs that play in-game, and on the menu. Originally created by N4fy, they were added on January 29th but there was a problem so it didn't get implemented until a recent update which fixed it. You can turn off music by pressing the MUTE button on the main menu.
There were two types of music, the menu and the gameplay. Each has its own thing. The menu sounds exciting while the gameplay sounds scary.
The menu tune sounds like a wondrous/exciting animal walking around in a forest (because mope.io was believed by many, a forest) that makes you feel to be happy and not to rage.
The gameplay tune sounds mysterious/scary (but it's fun to listen to!).
- If you went into the Sandbox on January 29, which was the day where it didn't work, the land song played for a few seconds before the update message showed up.
- After the Feb 10 update the gameplay tune only plays for 1 second.