Syrinscape uses the super powerful Unity engine for its 3D surround audio. At the moment that's what's stopping it from going into the background. Think of Syrinscape more as a fully fledged sequencer than a music player playing a single mp3. Often Syrinscape is playing back 20 or more samples simultaneously, each with dynamic positioning, reverb and even doppler effects.
I'm not sure how the remote/master feature allows for background sound, so I will try to explain my use case.
While GMing, I only use my iPad, so I use Syrinscape from there. If I change to another app (check some rules for example) Syrinscape stops playing. I've been able to avoid it by using the multi view introduced in iOS recently. but that forces me to use Syrinscape always in at least half my screen (the stretching for different aspect ratios is terrible).
The expected behaviour is for the music to keep playing even if I play the home button to check another app.
As you are using Unity, here's some info on the subject. Last time I tried it in a test project it worked.
Yeah. Looks like we got background playing sorted for the next version of the Players! Yay!
Plus a few other really useful features people have been asking for for a while = a volume control for OneShots.
Any estimate on when that version will release?
Later than I want it...
*jumps up and down on the spot