to change the audio source you have to manually restart the stream
if u're capturing the entire system audio, other users in the call will hear themselves which isn't ideal, but i think this is a venmic issue as this also happens on vesktop/equibop
Canary is up to 0.0.44 I would update to that as it has changed a bit.
Changing audio source has already been reported here
https://feedback.fluxer.com/p/549
There are extra options for the audio sharing like the mic fix, and make sure you aren't using any mic filter apps like easy effects as that causes a loopback problem with the venmic screenshare audio.
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Comment by @pollux78