I would like to be able to combine an external signal (available only via headphone cable) with the the audio out of my iPhone and listen to both of them using headphones. I know this is possible with an external mixer, but I'm hoping the folks here my have a better way to do this. Maybe the phone can take audio in via the lightning connector or bluetooth and merge it? Or maybe there's a tiny purpose built mixer for this two headphone-level input use case. I care only about the music player on the iPhone, or possibly streaming music like Pandora. I don't need actual phone calls or anything.
Yes, it's called Audiobus, in the Appstore you can find multiple AudioBus compatible Apps, you can choose multiple source and let them stream in compatible apps or on the headphones or other outputs available on your iPhone.