As John says, I also believe the down converter provides the current to run the 12Vdc components. When the load exceeds the down converter capacity, the extra current is supplied from the 12V battery. This all works if the average load does not exceed the down converter capacity. If it does, the battery will eventually be discharged.

So, make sure your audio system does not present an average load greater than the down converter capacity (Amps).

....you could custom build a FET or tube amp to connect to the HV battery. That could really make some noise! (Only for you design engineers with too much time on your hands).

2008 FWD Escape Hybrid