Android - how to have two applications play sound simultaneously?

As for my device, whenever I play duolingo and music at same time then the audio fades slightly with CM 12 built-in player

Now if you are OK with any third party application. Here is it what you need to do.

1. Install Poweramp Music Player .
2. Go to its settings.
3. Then click on audio.
4. Select Audio Focus.

5. Uncheck Short Audio Focus Change.

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You CAN play two audio simultaneously. I don't think it's possible with the stock Music player. You can install a third party music player. In Rocket Music player you can configure the Audio Focus to ignore the other audio apps while this player is playing. You can do so by setting the option under the Rocket Player Settings > Sound > Audio Focus,

_{Screenshot - Rocket Player (Click to enlarge)}

While I tried to play a song in Google Play Music app and played a song in the ESFileExplorer the Play Music song stopped. But while doing the same in Play Music and then in Rocket Player the songs never paused. It both played simultaneously.

Samsung sound assistant does this and allows you too adjust volume of both apps separately