Beginner question: Finally I got the “Spectrum Shield” with the MSGEQ7. 
Now i have to solder the connectors. But: How will I solder both layers of the PCB properly to the connector while one side is covered by the connector itself? For example pin D4 is connected by the lower layer, D5 by the upper layer.
The solder will flow through the hole via capillary action… easiest to watch a few videos on YouTube but the flux in solder reduces it’s surface tension and by sufficiently heating the pin being soldered it should flow up just fine.
Plus, the holes are plated… even tracks on top are connected hence the name PTH or “Plated Through Hole” …
That shield has two of them on it right?
Yes - one for each channel. P.S. Got it connected and working now…
The holes in industrial produced PCBs are
metallized on the inside like vias. It is not necessary to soldering each side. Only needed for a better stability. But depends on your connectors hardware.