I seem to have done something to override the native audio controls. When you’re in my (Android) game, I would expect the phone’s hardware volume buttons to control the game volume, and show the operating system’s volume slider. However the pressing the buttons does nothing. The volume remains unchanged, and nothing appears on the screen.
Answering my own question for the benefit of the community. It seems that just like with the back button, you need to explicitly handle presses on the hardware buttons on Android. The following tutorial has sample code that works to control the master volume.
https://coronalabs.com/blog/2013/03/26/androidizing-your-mobile-app/
Kudos on the self-answer and solution share. 
Answering my own question for the benefit of the community. It seems that just like with the back button, you need to explicitly handle presses on the hardware buttons on Android. The following tutorial has sample code that works to control the master volume.
https://coronalabs.com/blog/2013/03/26/androidizing-your-mobile-app/
Kudos on the self-answer and solution share.