ok my idea about this:
Since the AMD GPU seems to be the main, and it is hardwired to the internal card, I would say only the internal Soundcard can be addressed, as long as you use the amdgpu as main.
If you need the Nivdia Soundcard for the HDMI output, which must be hardwired to the Nivdia GPU, you have to use the Nvidia Card as main somehow.
Maybe you can switch it with the optimus-manager? Since I have no laptop with a hybrid GPU I can not assist here.