In Polyphone 2.4.1, when I insert headphones into my MacBook Pro, sound is still heard through the speakers. I'd have to go to settings and select headphones as the new audio output. Then I unplug the headphones but they are still shown as available audio output in settings.
My suggestion is to prompt the user if they want sound to be sent to the newly inserted headphones. This behavior is akin to other software such as Logic Pro X.
And when headphones are unplugged, settings should not show them anymore, and output should automatically and temporarily fall back to built-in speakers (default route if any?). Temporary fall back means: When restarting Polyphone, settings should use the last configured audio output (in this example headphones). If not available, fall back to speakers but don't save speakers in settings.