A Pipewire service hasn't been added yet. I started working on one a while ago but haven't got around to finishing it. Pipewire is a user service, so if you disable pulseaudio and run Pipewire from a terminal you may be able to get it running manually.
I'll have another go at making the Pipewire service soon. The last issue I ran into is that since Guix doesn't use systemd, there is no socket activation support that Pipewire can use, so I wasn't sure how to launch it.
