To my knowledge, you're not going to be able to do this without re-encoding the stream, unless you had a specialized client app that specifically allowed this.
The easiest way might be to setup a VLC server to pull a stream and then re-encode it with an audio track. That would also give you the benefit of keeping the camera viewing clients from directly accessing the cameras.
Also, I'm assuming these cameras are in enclosures? It *might* be possible to get a small solid state MP3 player and embed that in the enclosure. Feed the audio output to the cameras audio input (also assuming the audio in is currently not used). You could power the MP3 unit off of the same power as the camera, or with a PoE splitter if it's powered via PoE.
IPVMU Certified | 11/12/15 10:00pm
What about the Axis P8221?
Louroe Electronics | 11/12/15 10:03pm
Are you essentially looking for a wireless IP mic?
With that camera you don't need a powered mic but still have the dilemma of a short run of stereo cable. Can you add conduit between a mic and the camera?
Put an iPod inside the camera housing and connect it to the audio input of the camera :-)