I always make an extra effort to upload the videos on Y0utube with good sounds quality (320 kbps).

Is that discounted in the total video (say 45 mbps)?

Or is it video + audio?

Point is that I use great audio and it eats some video.

In 4k it might not make that much difference but in low resolution...hmmm.

What do you think?

YouTube will transcode your video again once you upload so I wouldn't worry too much about the bitrate of your video being 45Mbps.
They will transcode at a much lower bitrate. Leave the sound at 320kbps as I don't think they transcode the audio so heavily so you're starting with much better quality audio.

Use something like 4K downloader to rip a copy of your video back from YouTube and you'll see even at 4K the bitrate is significantly lower than your original.