Why did sound stop outputting to my computer monitor?

I got a PS4 and a new monitor, which worked quite well together for about a month. I was using a relatively cheap HDMI switch to switch back and forth between inputs.

Then, all of a sudden, the monitor stopped playing sound from my PS4. I switched out the cables, and made sure that the monitor still played sound from my PC. I also checked to see if the bluetooth controller was still able to play sound through it's headphone jack, and everything is still working there.

It's got to be something wrong with the way the PS4 is handling the HDMI, or the HDMI port itself - but what?

Furthermore, the monitor no longer recognizes that it is receiving input from the PS4 when I switch cables, or unplug the PS4 HDMI cable and plug it back in. The PS4 is still on, but the monitor is not receiving any signal.

What's going on here?