I found the following workaround on my Mac minis (OSX Mojave) which might help finding a permanent solution:
We have a Behringer USB audio-interface in house. When I plug it to the mac Mini, the system has an input-device (Which can be seen in Preferences) once it is plugged. Then, Sonic Pi starts and can be used. Even if the interface is unplugged once it is up and running.
With the USB interface not plugged, I get the same error and SonicPi can’t start.
Plugging phones or an external speaker has no effect on SonicPi boot behaviour on my system.
Any ideas how I can prevent SonicPi from waiting for an input source? (I don’t need it)
Or any ideas how I could add a permanent “dummy input” source to my system that would make SonicPi start?