Music is frequently played by moderators. Although some players like it, others find it annoying and disturbing.
To disable the music add the lines "127.0.0.1 www.mousedj.com" and "127.0.0.1 trans4mice.com" (without the quotation marks) to your hosts file located at "C:\Windows\System32\drivers\etc". This will block access to sites hosting music for Transformice. You might need to restart your computer before the changes are applied. If you want to hear the music again just remove the line from the hosts file