No sound in earphone

I have noticed the same problem as you. When I use Synfire and change an audio output, the sound stops. A solution that might help you ( this is what I didi to solve this problem) is to choose another output and then to choose earphone output again. So, you have not to restart Synfire every time you change an audio output.

The issue is, when the earphones get pulled, the Mac makes that audio device simply disappear. So if the Audio Engine was setup for that device, it's no longer available.

Synfire does not automatically switch to the (default) speakers device, when you pull the earphones out. There's a lot happening when the output device is changed. If the sample rate or buffer sizes change, all plugins need to be re-loaded and re-initialized, which can take some time. Therefore you need to setup Synfire for the device you want to use and stick to that.

Still, I remember vaguely the Mac can setup a custom device of sorts (check out the "Audio MIDI Setup" app) that spans multiple I/O devices. Maybe that's a possibility to setup Synfire with a device that handles both speakers and earphones?