Anyone know of a product that will mix an audio signal onto a composite video signal to transmit them both together on the same coax cable? On the other end of the coax we would need to seperate the two again.

Del, the Panasonic Model WV-CP180 combines audio, video and power over a single coax and combines the SDIII (Super Dynamic) technology.

A typical System would be as follows:

Eight (8) WV-CP180
One (1) WV-PS158

I've used this before with good results;

http://www.fmsystems-inc.com/prod_list.cfm?cat=cctv&industry=none

Product code CCR412

I've also used the FM systems unit and it works beautifly.