I fixed audio stuttering by disabling MalwareBytes (premium with the real time protection).
So maybe you guys can look into that too on your side 
It's thanks to the idea you mentioned above, basically making sure that the ports are open and Multiplicity is allowed by the firewall. That gave me the idea of pausing MalwareBytes and it immediately transferred the audio smoothly again like magic.
You can try it to reproduce the issue.
Cheers