Something changed recently and I lost audio (but not video) with my Macbook air connected to Dell 4k monitor with powered speakers.,

Macbook air lost sound through my Dell 4k U2723QE monitor (with audio out going to powered speakers.) In the Apple Sound menu, it only lists my monitor with DP instead of USB-C. Has Apple forgotten the Macbook Air has no DP output?

Not sure of exactly when the sound stopped working as I was learning piano online through youtube and stopped for a few months. Picture is unaffected. Just no sound anymore. The only workaround is to play the sound through the Macbook's speakers while watching the Dell monitor. Dell had an update which I installed but to no avail.

Why does Apple's Sound menu only lists my Dell monitor with DP instead of with USB-C (as that's how it's connected.) I'm thinking this is where the problem lies. Video is being sent out on USB-C but not audio. (I have 3 USB-C 8k cables and I still get only video without audio.