Arduinos for 12v system control?

But maybe I'm reading the program he has running on the screen wrong, it is wobbly and poor image quality, and he is actually using the SGTL5000 chip, and not the Cortex M4 itself.

It should be noted that the SGTL5000 is not meant for any even remotely audiophile applications. It has very basic functions and the processing power is limited so output quality could hardly be described as more than "adequate". 

In any case, I think it's an extremely poor and very expensive solution to something that can done much better and for much lower cost using for example a TI5756M chip.

Even if you're not using the amplifier section this gives you a fully and highly capable GUI programmable DSP when paired with a simple low power, low cost µP.

Now, I know there's no actual products available on the market today (that I know of) using it or similar chips from other manufacturers but maybe there are people working on that (hint hint).

The common part about all these though is that they have no analog inputs, they have analog output amplified or bypassed to line level, but the data capture (if any) is handled off chip for example in a bluetooth receiver chip which commonly have a very decent quality stereo ADC built-in and can output digital signal from either the wireless connection, the ADC or in most cases also directly from USB since it can be used as a USB audio device.

As for cost, well, yes I'm sure the Teensy + audio shield costs a lot compared to the chip you mention which is probably a few pence, but the fact is it's still not $100's, and I really haven't got the expertise to produce my own PCB's for surface-mount devices, so unless the TI chip was available in a similar format to the Teensy (already on a board with all important pins brought out to full-size and some basic power and programming hardware) I wouldn't really be in a position to use it right now. And I do want to finish this project before the end of the summer :)

My colleague's got a Teensy board, so first I'll be working out how to make that do the power sequencing and level metering, then perhaps I'll look into the audio side of it.