I'm going through a bit of an insomniac period at the moment and am listening to lots of radio programmes on my phone at night. I've found that I often drift off to sleep while stuff is playing but get woken up when it stops. I decided that it was the sudden change in volume that was waking me. Looking at the play store for "fade" apps, I couldn't find anything that did quite what I wanted. So I wrote my own volume fader.

This app lets you set your device to fade the media volume to quiet over a specified period, starting after a specified delay.

It seems to help me to get to sleep. I hope it works for you.