If you do, please link to it here so those of us that are interested can follow along!
Will do after making some progress.
I'm not much help with the sensor values (thankfully I think others have given you solid leads), but I did have an idea on where to hide the speaker... There's a sort of "exhaust port" on each side behind the rear fender. It's where air escapes when you close a door/hatch. You could probably sneak a speaker wire through it and mount the speaker nearby. Routing the wiring to the back might be a pain though.
Thats what those things are for! pressure equalization... Also, I think that's a great idea.