Ancient yeah, haha. Glaces at win311 and slackware 2.3 VMs. But yeah it does require guest addons but is probably going to be the closest to built in support you’re going to find. I think there is a novnc audio project but have no idea what state that’s in and what you’d have to do to roll it into the Proxmox novpn version.

Spice may be your best bet. I think it supports audio.

*Might be a good starting point https://forum.proxmox.com/threads/noob-guide-to-getting-audio-to-work-in-linux-guests.127369/