Apologies, as this wasn't as good as I would have liked it to be. I just realized my new laptop doesn't have a firewire interface, so I had to use the only USB mic I had available.
Methodology:
USB Directional microphone positioned on the glass above the plunger lane. Directionally pointed at the left flipper
Mac OS 10.8 with GarageBand
I recorded with the BezelGlass and a standard sheet. Upon starting the game, I fired the left flipper a few times, followed by the right, followed by both. I then plunged the ball, focusing on the center ramp shot. The first 25 seconds or so will probably give you the best feel for sound differences.
Bezel: http://dl.dropbox.com/u/2113522/damepning/Bezel.mp3
Standard: http://dl.dropbox.com/u/2113522/damepning/Standard.mp3
I'll snag a work laptop this week and do this more properly by next weekend. That said, I think the files show there's a negligible if any difference from a sound dampening perspective.
Edit to add: I've drafted up a couple of thoughts about the glass after trying it out for a while across the different machines. I'll post those after PZY posts the video, as I don't think it's fair to undercut the work he's done in cutting all that footage together.