Pinball Repair - New Speaker 

Sorry, I'm more of an early-80s Bally guy. But I watched a video on your machine and it looks amazing! Whenever I get an issue like that, I try to isolate exactly what happens to cause it. Is it some simultaneous hit? Does something open up during multi ball that is unavailable or rare otherwise? Then try to reproduce it, without having to play all the way to multi ball! At that point you can begin looking closely at the switches or solenoids involved. Tracing the schematics at that point may help. The Pinside community often has solutions to problems already, or join and post your problem there. I always find that, when something goes wrong, I get a hopeless feeling in my stomach that says this may be it. But I've always been able to ultimately find a solution!

Another thought; go over any changes or fixes you've made recently, and make sure they are working as expected. Any solder spills on anything? I keep a journal of all repairs for that reason.

Here's a Pinside conversation on a related topic, with some ideas for tests. I hope it helps pinside.com/pinball/forum/topic/testing-de-ball-trough-switches