OK - first question: have you changed skins recently? Some of the custom skins don't include a blue icon to indicate that a direct audio path is being used.
If the Audio Path status says that the output is Direct, and you're not seeing the blue icon, that's the most likely culprit.
Try changing your skin to the default Noire.
HDMI output is digital so there's no need for an HDMI "sound card" - a sound card is typically used to convert digital signals to analog.
Nvidia's HDMI drivers seem to be very stable/reliable for me, the only thing is that their hardware lacks the option to output 88.2kHz or 176.4kHz, when I think AMD and Intel can - though I have no idea what their drivers are like.
There are no HDMI devices that I know of for PCs which support DSD or DoP though, which is something I'd like to see addressed.
The "Automatic" bit-depth will use the highest bit-depth that your hardware supports.
Over HDMI this is 24-bit, and most (all?) HDMI interfaces use 24-bit in a 32-bit container rather than a plain 24-bit signal. ("padded") As I understand it, that's due to the HDMI spec, and not the video cards or drivers.
As long as the bit-depth is equal to or greater than the input, you have "bit-perfect" audio, and the audio path will be marked as being Direct. (blue icon)
Manual Volume adjustments or Volume Leveling should not affect the blue icon, as these are non-destructive until you get below 48dB of attenuation with a 16-bit source and a 24-bit output, for example.
Adaptive Volume in the Peak Normalize mode will also allow the blue icon to activate, as you're only raising the volume and increasing SNR. In the other two modes, you are compressing the audio, which is not "direct".
Any other DSP functions will change the audio path to not being direct.