It's the entire sound board resetting, not just the music.

All DE sound boards play a line when they boot. On Simpon's its 3 seconds of the theme. On Star Wars it's Obiwan saying "May the force b.." On TFTC it's "No Pay-em, No Ga-em"

Once the sound board reboots it is ready to receive any sound or speech or music commands and will play them right away. However the sound board only gets the signal to play music as the start of the ball and only gets a new command to play music when the game wants to change the song playing. So a reboot in the middle of a ball will usually end up with the sounds and speech and no music until you lock a ball or start a multiball or start a new ball, something that will cause the MPU to send a new music signal to the sound board.

As to how to fix it, try reseating the connectors on the board. Usually a weak power supply tends to make the DE sound boards reset rather than the game MPU so new caps and bridges on the power supply might work.

I would first look at the power supply, measuring your 5v and 12v. My guess is that you have bad caps on the power supply or connector issues.