Can I do it in my garage or should it be in my driveway? This is completely new to me, I’ve never used anything I legitimately need a respirator for.
Yeah, you can use it in your garage with the door open, but like I said, definitely use a respirator. The minute you smell the stuff you'll want one.
Do most people do the inside of the cabinet too?
Some do, some don't. Just a personal preference, really. I didn't, but kinda wish I had because I think it looks better.
I assume if it is a good sealant i wouldn’t want to use it on my routes edges either?
Why not? I don't think it would matter either way.
I would also assume it’s not needed on any place I will have artwork?
Well, you'll adhere your artwork to a gloss or semi-gloss surface whether it's paint, polyurethane, lacquer, etc. Your topcoat will help determine what primer you go with, but BIN can be covered with virtually anything, which is one reason it's so great.
Finally, for the board for the speakers, I assume I can just “paint” it on with a brush around the speaker “grates,” or is that wrong?
If I'm not spraying it I use a high-density foam roller and throw it away after use. You can use a brush and clean up with ammonia, but that's kinda annoying. Also, if you use a foam roller you'll get a small orange peal effect, but no brush marks (I HATE brush marks).
It might just be the pictures, but it looks like the grain that's showing through on that panel is pretty pronounced. I doubt that 2 coats of primer and sanding will cover it all up/fill it all in. It'll help, for sure, but my guess would be that you'll need more coats to make that grain disappear. What kind of ply did you use? Sorry if you mentioned it in an earlier post...
Oh, and why do you need it tinted so dark? Even if you put it on white you'll still be able to cover it fine in 2 or 3 coats.