Just made it for myself recently, thought I’d make it available for others as well at a nominal one time fee as I searched long and hard and found nothing that would help, 
almost had to abandon Corona till Enterprise came to the rescue.
You need an enterprise subscription to make use of the plugin, I’ll provide basic instructions to integrate it + lua example code to run it (iOS enterprise quick start tutorials are amazing for new developers to learn using enterprise, hats off to the Corona team).
Is as simple as:
audioDuck(true)
audioDuck(false)
Just call it in your sound/music player as required.
Disclaimer: Not heavily tested.