The original first run designs worked well with 2.4ghz.
I did an entire JJC diesel sound and smoke unit install in a Tito Neri back in 2006.
Everything preformed flawlessly, and we were using a Spektrum DX6 at the time.
Second generation units developed a skip, and no one could get any sympathy regarding a fix.
Buyers were told they were being too sensitive about the sound cycle. However, I had purchased one of the original units for myself, and a second unit two years later, and when
I hooked each one up to the same radio, and same power and speaker out put, the newer
unit would heavily skip at full throttle.
Unfortunatley, I can't speak to what might be the issue with your JJC sound unit, unless you
managed to get a SLAVE unit, and are trying to run it unamplified without the master unit.
You did say that you have the V8 master unit, so it might be one of the POTs on the board need adjustment for volume. Facing the POTs, the right, outermost POT should adjust volume.Not sure why you might be getting interference.