Sometimes hollow boards can sound good, and other times they sound bad. This seems to be the latter. Is the interior parallel with the plate and pcb? If it's not, that could explain the sound.
This is a cross section view of the board
The PCB and plate are parallel with the bottom case. I wonder if using a 1.2mm PCB has anything to do with it? Although I'm skeptical of how much a .3mm difference would make in terms of sound. I think the biggest factor to the sound might be because of the keycaps. I'm planning on making another typing test soon with PBT caps instead.
What plate material are you currently using with that prototype, aluminum? I honestly don't think that sounds bad. Just an idea but, I'm curious if variances in the how "hollow" the bottom half of the case is from the inside could affect the quality of sound. Since the board has such a high angle, there is a lot more material on the bottom case to work with. If anything comes of it you could even go to try providing more of a cavity behind the alphas similar to the concept of a half-plate. Granted, there are a lot of other factors that play into sound quality such as switches, keycaps, plate, case material, etc.
FYI, I am only hearing sound out of the left channel on the video.
The plate is 1.5mm brass. I use a 'standard' 9mm distance from top of plate to bottom case. The bottom case only has a depth of 3mm. Are you saying that you think increasing this depth would improve the sound? My thinking is that that would make it sound hollower, but I don't know how sound works at all so you might be right.
Also I'll fix the video soon, I didn't catch that the audio was only on the left channel >.>
