I ran into an issue with my MacBook Pro where no matter what external audio output I selected, sounds would always play though my internal speakers. I stumbled upon this answer on the Apple Community forums where a user had a similar problem with AirPods and turns out this works for other external sources as well.
It appears that MacOS can get confused and not know that other outputs are selected. To get it working again you need to do only the following:
- Select the audio output to be the internal speakers
- Select your audio output to be another source
- Success! Or, if you still have issues you likely need to contact Apple.
That’s it. I experienced this on a 2017 15″ MBP running MacOS Catalina. I imagine that as the issue in the forum post was from 2017 running Sierra that this may work on all OS versions. As this solution was harder to find, I hope this was able to be found and indexed high enough that it can help others. Or, if you use MacOS, share with other Mac users!