I discovered that the option 'In The DJ's Mix' (in the channels configuration) is only affective when the respected channel is a) opened via a button on the main panel and b) this button is set to be treated as a microphone opener.
Is this behaviour desired?
I have a use case where I want the channel NOT to be present in the DJ's mix while it is not being a microphone channel and is to be open all the time (i.e., not being assigned to a main panel button). [An external DJ is connected via internet and (due to latency issues) should not get his own voice/music in the DJ's mix which is used as audio return.]
Otherwise, if the external DJ wants to contribute to a VOIP call, his audio must be treated as a microphone channel - which is perfectly reasonable. So my initial use case might not exist anymore... Sorry!
It is still a bug since it doesn't do what a user would reasonably expect so I fixed it.
Users can now mute auxiliary audio channels from the DJ mix with the possible exception that when in red phone mode and no microphone is selected the DJ and the VOIP callers can get to hear the stream mix as part of the VOIP sound source.