Gabor, I am sorry to hear you are having problems with your system.
I was checking the BIOS versions available for this motherboard on the download center, and there are 29 different files.
You said that you update the BIOS from version 0816 to the latest which is 1754. Technically speaking, going straight to the latest version shouldn't be a problem, but in the practice and based on experience, there is always chance.
Since there are many versions between one and the other, I would recommend you to roll back to the original BIOS version and then install or flash as many versions you can.
It will be worth it investing a little more time running some updates before the latest.
These are the BIOS versions that included fixes related to audio issues, for example Version 1162, 1612, 1669.
I would recommend checking the BIOS Update Release Notes on this URL. You will be able to see the fixes and features included on each version.
These are some important versions you could flash on your system.
BIOS Version 1162
BIOS Version 1250
BIOS Version 1612
BIOS Version 1669
BIOS Version 1698
BIOS Version 1729
I guess I didn't make it clear in the original post. The problem with audio already existed with the original 0816 BIOS. I just did the update in an attempt to try to fix it. I haven't seen any new issues pop up with the updated BIOS, so it's unlikely that winding back will solve the issue. I'm beginning to suspect that there is a hardware issue with this motherboard, but I'm not willing to debug it much farther at this point. Generally I don't use the audio, but without an audio device I have trouble with things like watching videos (even without sound) because the player expects an audio device. I was hoping there was something else I overlooked that might cause the audio hardware not to show up in the device manager.
Gabor, my suggestion will continue being the BIOS updated on the board from the original BIOS version to the latest one, installing some versions on between.
After flashing the BIOS, you can install the latest audio drivers for your motherboard. Download Center
If these do not fix the problem, please contact our customer support department for additional information on replacing your product if it is still under warranty.