This is a followup to this question. Since I'm not on the mailing list, maybe someone else can reply to that question with a link to this description.
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What we want to achieve: record audio output of qube A in qube B.
All instructions should be executed in dom0.
Run sudo modprobe snd_aloop
In pavucontrol, in the "Playback" tab, change the output for A to Loopback Analog Stereo.
In pavucontrol, in the "Recording" tab, change the source for B to Monitor of Loopback Analog Stereo.
Attach the microphone to qube B via either the applet in the system tray or using qvm-device mic in the command line.
The audio output of A should now appear as the microphone input in B.