What mp3 player app is this? How do I choose another app as default?

When I click an mp3 from either Downloads manager, or from OI File Manager, then the mp3 player that pops up is this one:

(I wanted to include a link to a screenshot here but the forum complains it looks like spam.)
How about this: http://ift.tt/2aVBriu
The app looks like a small, plain, white window that only covers about 20% of the screen, with a horizontal orange progress bar, and the mp3 track name in black text below that, and a black play/pause icon next to that on the right. Nothing else.

I want to change the default mp3 player app to something else, but apparently to do that I have to find this app in my 'settings/Apps' - and I can't find it. It doesn't seem to be Google Play Music. When I go to Settings/Apps/Google Play Music/Opens by Default/ it says:

Open supported links: ask every time
Supported links: Open music.google.com and related URLs
Other defaults: No defaults set

I don't have any other music/mp3 related apps installed (except the brand new app I just installed, that I want to use as default instead of whatever the above screenshot is.)

Does anyone have any ideas what I should try? Thanks.

I could try deleting the 'open' prefs for every app I have installed, but, yuck.