I think you could probably change your audio output back to the headphones from the iOS Control Center, but there's no simple way to mix incoming audio with app audio and send it to the headphones. You'd have to run a mixer app that supports inter-app audio, and route the app audio and input audio through that. You could try GarageBand, but I don't know if it would be worth the effort.
The other option would be to send the app audio out to the FastTrack, mix it with your guitar audio there, and connect your headphones to that, but then you wouldn't be able to use Bluetooth headphones.
Maybe someone has another idea?