I don't have too much to say about this except for the fact that it works as expected, and the sound device did not require a reboot. I have a spare PCI-E Sound Blaster Audigy card that I used for this test, I would assume that a USB sound device of some kind would function the same. When I first created the second seat and logged into KDE Plasma, I was greeted with this:
After assigning my motherboard's onboard sound device to the second seat and logging in a second time, I was greeted with the following image. I plugged in my audio cable, and indeed it worked for its assigned seat. Sound on either seat was independent as expected.
EasySeats is open source software developed to bring ease to users who choose to go multi-seat. It supports video cards for seat creation as well as external USB docks.