Here is my thoughts on the voltage regulator and capacitor.
Why not?? Basically, the voltage regulator is designed to give a constant 5v output on a range of varying inputs (i think 6v-18v). Then, when you're dealing with the noise later, you can drop this out of the "what if" questions.
You cannot use a diode because the audio signal that also passes through keeps switching from + to -. THe diode would block out all the -! Instead, the capacitor blocks out DC. Anyway, some soundcards (from what I read) do not use the computer ground so you could potentially hook it up wihtout a capacitor.
As far as the mp3 player staying on after you already turned on with the HU... R32 reported that the 5v turn-on voltage is just that... a turn-on! After you apply it, you can remove it and things should still work. I tried with mine, but it doesn't seem to work as well without it... the sound quality degrades... haven't figured it out yet.
But one more thought...
I am still hearing Hard-drive and engine noise on my speakers! And it's driving me crazy now. Anyone have any ideas on how to deal with this? I tried an RF choke, but that didn't do anything.