iMac can't play videos or iTunes

iMac (Retina 5K, 27 inch, 2017) running macOS Mojave.

I am frequently having an issue where I can watch and listen to streaming content (YouTube/Netflix/Spotify), as well as listen to my iTunes library, for a couple hours before nothing works. I restart my iMac and everything functions as it should for the next 2-4 hours. Then I have to restart once more. This happens multiple times a week.

YouTube videos will try to load until a message appears; "If playback doesn’t begin shortly try restarting your device."

As for iTunes, I can press the play button, but the scrubber doesn't budge.

Spotify: the scrubber moves along, but no sound is playing.

The only bluetooth devices I have connected are the mouse and keyboard. I've uninstalled and reinstalled Adobe Flash Player twice and nothing has changed. All extensions have been removed. Nada.

This is nothing but an annoyance and I doubt this warrants a trip to the Apple Store just yet, but I just don't know what else to do. If anyone could help me identify and fix the problem, I'd GREATLY appreciate it.