Speaker distortion after Logicboard exchange.

I changed the Logicboard of the iMac. Since I did this the speakers are not working properly. If I plug in both speaker cable then the right speaker (looking as user) ist making distored sounds already on the startup chime. If I plug out the other speaker, then the right speaker sounds good! If I plug in just the left speaker (the one with the subwoofer), then no sound at all.

What could cause this problem? I changed form a i5 board to a i3 board and replaced all my i5 items (GPU, CPU, heaters). But I don't suspect this to be the issue.

I checked the Ohm on both speakers. Between 1-2 it is 5 Ohm and between 3-4 it is 10 Ohm. This is true for both speakers. So, no short circuit apparently.