New answers tagged speakers
This is easily done with an Aggregate Device as described here https://support.apple.com/en-us/HT202000 Basically you combine multiple input/output devices into what appears to the OS to be a single device.
I believe Audio Hijack from Rogue Amoeba does this, but I'm not certain. They have a free trial, so you can make sure that is possible before you buy it.
I just worked it out. The entire time Bluetooth was on. Usually when I turn off my car the phone automatically disconnects. I guess this time something went wrong. Even after restarting the phone away from the off car the phone still thought it was connected. All it took is to turn Bluetooth off
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