Just FYI for other people reading this…
I have got setup my plexamp as follows:

• Basic HP G2-800 mini-pc machine ($100)
• Proxmox 8.03, using an LXC containers with Ubuntu server 22.04
• I have one container for Plex Server, another for Plexamp
• And then audio output from Plexamp via USB dongle (or line out works too) into my amp
• I also have a VM with home assistant on the same box, that turns my amp on/off via smart plug when plexamp is playing.
• Final result is that I can open plexamp on my phone, select the remote player, and amp turns on and music plays! Amp switches off automatically. Perfect!

A few details on steps below - hopefully this helps anyone else wanting to do the same thing.

Adding the following lines to your lxc container config allows the host soundcard to be accessed within the container:
use:
Type into host shell: nano /etc/pve/lxc/<container ID>.conf

Add the following:
lxc.cgroup2.devices.allow: c 116:* rwm
lxc.mount.entry: /dev/snd dev/snd none bind,optional,create=dir

Then (if unprivileged container) you need to enable access to the soundcard group from the host within the container. A few ways to do this (e.g. user group mapping) but I just opened access for all users.
Type into host shell: chmod -R a+rw /dev/snd

I also installed Alsa on the host & container.
Use speaker-test to check if your sound is working on host, then on container
Only once you have working sound on the container, then install plexamp. I found after setting up the soundcard, the weird issues I had with running plexamp stopped and everything worked.