I looked over the design this evening, and sure enough found a couple of mistakes.

First, the TSSOP-24 footprints for the MUXs were slightly too narrow (I had used a 3.9mm wide footprint, when I should have used 5.3mm).

Secondly, the Teensy audio shield was colliding with the electrolytic caps. I moved those to be in a better place.

Thirdly (Tertiarily?), I did some minor re-routing to reduce vias on signal traces and hopefully make for a cleaner board overall.

I also ordered the faceplates for the enclosure (front and rear). I am planning on making a wooden enclosure for this project, similar to my other recent ones; however this one will not have an aluminum faceplate (for two reasons: 1, I want a lot of text on it to describe input function, and 2, the RCA jacks that I am using would all have the outer rings shorted together, and the outer rings are not all at the same potential (some are grounded and others are at 3.3v)). Instead of aluminum, I am trying out Ponoko laser cutting / etching for the first time. The price is right (at least until you add shipping to Canada - boo!), and I have a coupon for it from a previous HaD prize win, so it really costs me nothing.

The faceplates can be seen at https://github.com/thebiguno/microcontroller-projects/blob/master/projects/drummaster/rev2/enclosure/ponoko_faceplates.svg (you may want to import it into Inkscape or Gimp to see it better; the colors required for laser cutting / etching do not show up very well in the browser).

The board order has been placed at Dirty PCBs. You can order your own board in the Dirty PCB store at http://dirtypcbs.com/view.php?share=10126&accesskey=, or you can just look at the pretty pictures below.