CamStreamer App supports external audio sources via URL. If you want to use audio from another camera in the same network, use the URL rtsp://root:password@IPaddress/axis-media/media.amp?audio=1&video=0.
Works also with P8221 audio module as: rtsp://root:password@IPaddress/axis-media/media.amp?audio=1
In addition if you do not have an external audio source on hand, our CamSwitcher app offers the solution of streaming audio via multiple audio sources.
https://camstreamer.com/camswitcher-about