After my investigation, I found this info may be helpful.
In my old laptop (R4.0) the dom0 pacat-simple-vchan
outputs like:
Connection established.
Stream successfully created.
Buffer metrics: maxlength=4194304, tlength=3072, prebuf=2052, minreq=1024
Using sample spec 's16le 2ch 44100Hz', channel map 'front-left,front-right'.
Connected to device alsa_output.pci-0000_00_1b.0.analog-stereo (0, not suspended).
Stream successfully created.
Buffer metrics: maxlength=4194304, tlength=4096, prebuf=4294967295, minreq=4294967295
Using sample spec 's16le 2ch 44100Hz', channel map 'front-left,front-right'.
Connected to device alsa_input.pci-0000_00_1b.0.analog-stereo (1, not suspended).
Stream device alsa_input.pci-0000_00_1b.0.analog-stereo suspended.
However in R4.1 new machine something is different.
Connection established.
Stream successfully created.
Buffer metrics: maxlength=4194304, tlength=3072, prebuf=2052, minreq=1024
Using sample spec 's16le 2ch 44100Hz', channel map 'front-left,front-right'.
Connected to device **auto_null** (0, not suspended).
Stream successfully created.
Buffer metrics: maxlength=4194304, tlength=4096, prebuf=4294967295, minreq=4294967295
Using sample spec 's16le 2ch 44100Hz', channel map 'front-left,front-right'.
Connected to device **auto_null.monitor** (**0**, not suspended).
Stream started.
It seems that pacat-simple-vchan misunderstood the channels, or the protocol changed.
I am still unfamiliar with the pacat-simple-vchan tricks, and am asking for advice.