Is it possible to have two different audio sources go to two different audio outputs? For example, can I have Spotify output through my mini Jambox while having YouTube videos play through my headphones plugged into my audio jack?
Short answer is yes, it's possible; however, the application must allow this. Most applications default to the System defined audio output which is set in the System Sound Preferences (and can be further tweaked in the Audio/MIDI Setup app). Some apps have the capability to set there own independent output as well (for example, most audio apps (including Garageband) allow you to set it's own audio ins and outs (on a per channel basis)).