I have installed nvidia-driver-455 for GeForce RTX 3070 on ubuntu 18.04.5 LTS
After that, there is no sound over the headphone.
But before installing nvidia-driver-455, the sound over the headphone was working.
Please check alsa-info:
http://alsa-project.org/db/?f=bd609e4fbf9e108ce1b653fa491d7d0664b92010