Lab of the fusionist and the bug is more than just the music not working.
Sounds to me like you have may have not downloaded all the latest Music files. When music files are missing, it can affect more than just music not playing. Download the latest MKF V0.91 - MUSIC ONLY file and unzip it into your game directory.
The letters for the puzzle of one of the jigsaws does not appear. I cant pass the stage.
Known bug that will be fixed in the next update.