My employer is MTX Audio, we have a pair of Escalades each with two of our Jackhammer woofers installed and 12+ kW of amplifier power driving them. The older Escalade has six, 400A Powermaster alternators installed in it like this:
But that's a show vehicle, designed to exhibit a product... efficiency is irrelevant (though the 22" Jackhammer is an efficient sub at 97dB@1W/1m)
I think overall using an engine to run 12v alternators to charge individual 12v cells in a higher voltage traction battery isn't going to be anywhere close to efficient.