So having root as your only system user can be a problem. I’m not sure if it’s the cause your inability to stream audio, but pulseaudio could be confused. It’s used to running as a ‘normal’ user and may not like running as root.
Is having only root as your primary user a Proxmox default? If so I’m surprised.
At any rate, I found this tutorial by @GrayBoltWolf on Youtube for Scream. He usually works with Debian (Proxmox’s ‘parent’ distro) and I’ve used his Debian passthrough tutorial in the past to set up PCIe pass thru on Proxmox. I should note though that I had a full Linux desktop and dedicated GPU. Proxmox can be expanded into a full Debian desktop, or at least it used to have that capability. And an old crappy GPU will do for the Linux side of your solution.
