"Do I actually need some sort of audio interface separate from the cat interface?"
The CAT interface is a low baud rate serial interface for command, control, and radio operating info only.
There are a fair number of commercial audio interfaces available.
The Rigblaster Plug and Play has BOTH a USB serial interface and an audio interface, so it duplicates the function of your existing cable, but it does indeed seem to ship ready to plug in:http://www.westmountainradio.com/RIGblaster_P&P.htm
There are probably less expensive options available from other suppliers. Maybe some others who are more familiar with other offerings can chip in.