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.