Firstly, this can be quite easily achieved if you're running a device with Android 4.4.
I'm not sure if "App Ops" is available on lower android versions. But if you're on KitKat, you can simply go into App Ops, scroll through the apps, find you game. And turn off the "Media Volume".
This stops the game from using inbuilt music and sound fx. This means you can keep your music on in the background.
Hope this helped, if you need to ask anything else, please do so.
Sent from this toast making Nexus 5, you wish your Nexus could make toast don't you? (;
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