"ra": "NOT <== RECEIVING AUDIO FROM PEER",

[B.B.]

IMHO this is not a jkz issue but rather a networking/computing challenge. The jkz service establishes a peer-to-peer connection between the two of you. Did your friend try to create a dedicated portrange for the app to run at? Both in the app and per his router? (the in the app suggested 12000 and up are hardly used by anything else [but could be blocked by an ISP] )

Thanks Dimitri!
In order to get a better overview i'm trying to educate myself a bit at this point. So my questions are also:
Is the Rand_Port info telling the connections between ports? If yes, then it seems that we were connected via port 57629 - is this correct?
"rt": "RAND_PORT info:\n 0=57629\n 1=0\n 2=0\n 3=0"