Just reporting in on you, as promised.
Flashed the ROM, flashed GApps and flashed the baseband you suggested. Audio playback seems to be fine now!

Thanks a lot.
The only thing I've got to mention is that I experienced some random reboots while setting up my phone (language selection, network, google now,...). But at this moment the phone is fully up and running fine on CM13 build 20160416, open gapps stock 20160416 and baseband I8730XWAMK1 with stock kernel (gonna leave the kernel for what it is for now).
Lets hope the random boots were just some settling issue and the phone stays stable for the later.
Cheers and thanks again Penguino!