Okay...
'Any ideas' part... yes. The volume issue (or no volume) - is very likely a compatibility issue, probably not easily taken care of. IIRC the six-stackers (PA-760) were designed to work with an external amplifier for all speakers (so had no internal amplification); while the the single stacker (PA-710) were designed to run the speakers directly - i.e. internal amplification and no facility drive an amp.
We do know when people have tried to replace a PA-760 with an FD-01 (an Australian OEM bluetooth unit - which is pinout compatible with the PA-710 - for the 1st Generation body type) it doesn't work
Aust - FD i30 - Missing Outside Temp. Here is how to fix it! ... I suspect for exactly the same reasons as what you are seeing.
Unfortunately I don't know the pinout to the PA-760. If you want to use the PA-710 - then you might be able to bypass the amp by unplugging it (if you take this route be sure to totally disconnect it - no point it using power) and connecting the appropriate pins of the input and output plugs together. This assumes that the amplifier used with the PA-760 was feed analogue (line-level), not digital signals, - and will be easier if the pin function doesn't change between PA-760 and PA-710 head units. But I don't know the pinouts to do that (e.g. the PA-760 pinout and the pinouts for the AMP connectors).
As for the stacker with CDs in it - it will probably require disassembly. I am not sure if there is a reset button, or manual eject lever, you can try.
