Background music and the "bong" are generated on the remote sound board, not the CPU sound section, however the commands to that sound board originate from the PIA on the CPU board that is just below the battery holder. That PIA is controlled by the main processor, not the speech/sound processor at U24. If there are corroded traces this could cause some of the sounds from the background music board to not initiate or possibly initiate the wrong sounds. However that shouldn't cause the background sound board to reboot (the bong is the board booting up) and hanging on a note is likely that board locking up.
Inspect the small ribbon cable and the are on the CPU around the battery holder from the PIA to the cable. Reseat that ribbon cable at both ends and reseat all the connectors on the background sound board. Reseat any socketed chips on the background music board.
So typically on most pinball sound boards that generate music the following happens when it experiences a reboot. The command to play music only occurs once. Usually at the start of a ball or when the game changes states, i.e. ball in play to end of ball or after locking a ball or starting a new mode or starting multi-ball. So if the sound board reboots mid-game, it will make the boot-up sound (a bong for Williams games, a short speech quip on DE games). The board will continue to receive new sound cues so it will continue to talk or make sound effects, but the music will not restart until a change in state happens, i.e. end of ball or start of multi-ball.
You can see this on some (most?) system 11 games if you quickly press the start button when powering on the game. If you start the game prior to hearing the sound board boot-up bong, the sound board will have missed the start music cue and will then play most of ball 1 without any music. once you drain the ball the music starts on ball 2.
As PinballManiac40 said those Hitachi CPU chips (and Hitachi 6821 PIA's) tend to fail more frequently than the Motorola. Just note that if you change from the Hitachi 6802 to the Motorola 6802 I believe you have to change a jumper on the CPU board.