Much belated Update: I bought new batteries, and the light actually turned on! ...automatically. I can't turn it off. Speaker remains broken, looks like the board is busted, either that or the button is. Kind of an anticlimactic ending
Your commitment to posting an update on this thread should be lauded across the globe! Since you are in Germany, consider that achieved.
You get light (lit?) with batteries so the LED and batteries are okay.
Do you have a latching or momentary switch? Could be a short either by switch stuck closed, short across the switch ports on the board, or chip malfunction.
"Speaker remains broken" means no sound? visible damage to the membrane? both?
At this point, i recommend to gut the saber and explore all possible connections to identify the source(s) of the problem. You may want to take this opportunity to upgrade the components.
Also, be sure to join the Nintendo Fun Club today!