First things I'd do is a switch test. If any of those switches for the kickout holes are not registering a ball or the ramp switch doesn't get activated by a ball, then multiball won't start....but in order to get to the switch test, you are going to need to fix those service switches. Usually that would be either A) a problem with connector 1J14 at the motherboard, or 7J6 or 7j4 large harness plugs, or a broken wire on the switches themselves. The switches themselves are also common to become flakey.
As far as the display goes, try swapping displays around between the good and bad, if the good ones light up in the bad ones positions - then you know that the harness and everything is working. Check the display glass solder connections on the back side of the display boards, really common for the through hole connections to crack, and they can be tough to spot without magnification, or just reflow them.
Music options? not without some big modification, but I am unfamiliar with how that would be implemented.