Background game music can be changed via the Tavern Jukebox. Simply walk over to the Jukebox, click on "My Sheet Music", and then at the bottom, click on "New".
Once you're in the editing menu, you can either play around with the various instrument options, and input sheet music manually:
OR you can copy and paste the text version of sheet music directly into your Jukebox by clicking on the file folder icon on the bottom right, and this window will pop up:
A video summarizing how this is done can be found by clicking the image below:
Feel free to use this thread to share sheet music with others to enjoy!