I have looked at the manual of the PA. It is mono. The cable you have should work, but the output socket may be rogue and not work with a stereo cable like you have. In an ideal world the mono jack socket should connect to ring and tip, and give you dual mono at the two RCA plugs. However some sockets will be shorted by stereo plugs and I suspect this is happening in your case, as that PA system has a very nasty look to it.
Get a mono 3.5 mm to RCA plug and get a Y connector to connect from the single RCA plug to your two RCA inputs on your receiver.