On my machine I have two monitors on DVI, and two monitors on DisplayPort using MST and a DP hub. Previously, I only had one monitor on DP, and it allowed me to select that monitor as an audio output device.
Now that I have the two monitors, the hub, and MST running, I can no longer select the DP monitors for audio output. The default "sound" hardware settings don't recognise that it exists, and pavucontrol knows something should be there, but thinks that it's HDMI, and also unplugged.
In windows, the monitor can still be used as an audio device, but in Ubuntu it's lost that ability. Video works fine on all 4 monitors, just no sound from the monitor speakers.
e: Graphics card is a NVidia GeForce GTX 670, and NVidia driver is 375.82