Thank you for the compliments! 
Developers that only one “void” should probably learn to work a little cleaner, but that is just my opinion.
The Arduino “standard” dictates both “setup()” and “loop()” as the functions that always exist.
I was already wondering about the sound haha. I had never heard of DFPlayer mini – but did find them dirt cheap at AliExpress (here and here).
I may order a few of those. €1 is not much! And at Amazon.de they can be had cheap as well – but from the comments there I see one has to be careful what version to get.
I did find an excellent guide for this by the way at DFRobot.com.
One of these days I should get one … and a speaker with that haha.
Sounds like a major (cool) project you’re working on.
I’ll take a look at your videos for sure!