Welcome, Molitor.
We have the same issue as you have with audio. Check whether this decrease of the voice happens when you plug your player to some speakers or amplifier. It works well for me if I plug it into Creative PC Speakers, it is probably the same situation with you.
I am also curious where to look at ASM to explicit the right config for our audio problem. Please send me your FW (the one with audio working) to my email:
flasher86@gmail.com. It will be easier for me to compare 3 FW with 2 well working audio.
Disassembling is not a problem for me, I can identify almost every function which the SDK has, unless some new wicked structure functions has been introduced into FW by the seller.
The keyboard is also applicable for the SDK. It is easy to identify GPIO set of pins, so SDK may work for you in the future. Of course I am talking about standard buttons without some weird chip to control the whole thing.
Have you managed to modify already compiled FW by ASM? Or do you just work on the current SDK C compiler?