Hello Alphahurley,
yes, resin is more expensive - at the moment.
A few years back 1,7mm ABS was hard to get, now you get it everywhere for 30$ / kg and less...
As resin printers get more common, I am sure resin will get more affordable, too.
Optical 3d printing has (in theory) the potential for more precice parts and quicker manufacturing. We will have to wait and see how the Peachy performs.
Regular sound cards, such as onBoard sound chips, are rather bad. But I am sure there can be a inexpensive d-a/d board made to increase precision. We will have to see how exactly the mirror actuators will be controllable.

Protective phone cases are not the type of prints I would call high-precision, so I am certain it should work. But again, all educated guesses, right now we will have to see how developing of the Peachy goes, let's cross fingers :-)