Sadly, I'm afraid we're somewhat at the mercy of phone manufacturers and music app writers on Android. We do everything we can to make music "ducking" work on Android, and it works on "stock" Android phones from Google, e.g. on the Nexus series of devices and many others.
Sadly, some phones just don't seem to have the feature, and some music apps ignore the request to quiet down/pause the music even on phones where it normally works. There doesn't seem to be much rhyme or reason to it. I actually developed most of Get Running on a Samsung Galaxy device, where the ducking worked fine...
I've tried reporting this in places where I've found a problem, but with these behemoth companies like Samsung and Microsoft (for the Xbox Music app, which seems to have this problem) it's hard to even get ahold of a human to talk to, let alone get their products fixed.
Hopefully you'll be able to find a music app that does work for you—both Google's own Play Music app, and Spotify normally seem to work—but if you don't and that means you can't get Get Running working to your satisfaction, I'd be happy to offer a full refund, of course. Drop me a line at
play@getrunningapp.com and let me know.
Matt