Update on this project:
- I've separated both speakers and only conected one (I didn't try the speakers in serie after that): no improvement
- I've tried the mp3 player wihout having speakers connected: no improvement
- I changed the mp3 player for a new one: no improvement
- I connected the +5V and GND to Arduino (as before it was separate): no improvement
- I powered the system with only 1 powerbank: it takes a lot longer (around 2 minutes) but the arduino and the voltmeter shut off again as well.
Just to be sure, I changed the sketch and used the one with only the lightprogramming ... : it worked ....loaded again the one with the sound programming ... no luck.
Tomorrow one of our friends will bring her backpack, which has the "original" cabling, exactly as it was before I made any changes to it. I'll connect the extra mp3 player I have now and load the sketch again with the sound programming in it. That's how it worked (or at least I hope it does it again). When it does, I'll try to make a list of all the changes in hardware (if any) I have done to it. If it's not that, and it hasn't have anything to do with the sketch and the way I wrote/modified it ... then I'm afraid they will only have lights on it.
The best I can do than, is try to make a little sketch to operate with a pushbutton, the DIY smokemachine I made with a mini aquarium airpump and a vapertip .. ...