CamStreamer App supports external audio sources via URL. If you want to use audio from another Axis camera in the same network, use the following URL:
rtsp://root:password@IPaddress/axis-media/media.amp?audio=1&video=0
It also works with D3110 Connectivity Hub or C8033/C8110 Network Audio Bridge as:
rtsp://root:password@IPaddress/axis-media/media.amp?audio=1
Moreover, if an external audio source is not available, our CamSwitcher app enables streaming audio from multiple sources, including live commentary, similar to how browsers handle audio in online meetings
https://camstreamer.com/camswitcher-about