Needs some patience if applied the for 1st time for the plastic notches of the case are fairly stiff (it's a pure clamp construction).
Get a very thin, but slightly rounded spatula and start inserting on one side for a quarter inch or so. Then gently lever so the inside comes up a bit over edge. Do the same on the other sides and go round in a few sequences, lifting it more and more.
That's the basic idea, but check ifixit for details with the screws and some cables. It's not difficult, but a bit messy.
Btw my results are similiar to yours, and some even bizarre.
I don't have a matching external audio interface and onboard sound is either off or full on.
Initially it crashed right about on SonicWorx, then (after connecting the acryl speakers from a Cube) it played with crackles, next step was Koblo (that worked nicely) and eventually it's usable. Iirc Koblo's gain (running in background) could be abused to control general sound level, as the regular panel is disfunctional.
At least the Mini seems to have kind of settled regarding sound, but it's not 100% reliable.
Sitting on top of it's modern OSX bro is picking up a tremendous amount of noise from the OSX box, if that is active - probably because gain of the virtually non-existent input channel is full up. I expect an external interface to improve things much.
Strength and Honour to the fellows who made this possible at all and my deepest respect
ps: running it with an Eizo 1920x1200 - occasional screen 'going blacks' on the DVI input, but it comes back immediately. Seems to be the common ATI experience. This screen doesn't work with DVI from my G3 B/W at all (using a VGA to DVI cable solved that issue).