Updated the OP with a typing test
I'll be honest it doesn't sound that good
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.