DVI is not supposed to carry audio, but in practice in many cases it does. That’s because internally both devices are likely to implement DVI by just shoving an HDMI output through the connector anyway. The jury is out on whether or not this has any licensing implications. I’ll be damned if I know, because I was always under the impression that the part that incurred licensing fees was the HDMI port itself.

I rediscover this fun fact a couple of times every year when one of our office machines decides to randomly start piping its audio out of the monitor sounding like a mouse trying to play the kazoo through its sinuses rather than the speakers that are right there, and I somebody complains at me and I have to schlep over there and switch the audio output back.

Apparently this is expected enough behavior that cheap bottom of the barrel PC monitors bother to include speakers for it.