This app was mentioned in 10 comments, with an average of 1.70 upvotes
By the way, there's an app for android which lets you do it without turning phone upside down, just through normal phone speaker: https://play.google.com/store/apps/details?id=mobi.omegacentauri.Earpiece
If you would like to give front facing audio a try, you can give "earpiece" a look. It plays audio through the ear piece, just note that audio quality is generally pretty shit and not very loud using this method. Also note that the app has a "don't play music too loud via ear piece, this may damage said speaker" warning, and to play audio via the main speaker again, you may need to close all your running apps and reopen them a few times.
Linkme: earpiece
Edit: apparently I cannot into reddit, so here's the link for "earpiece"
This app seems to make audio play through the earpiece: https://play.google.com/store/apps/details?id=mobi.omegacentauri.Earpiece
This app lets you do it, but only through the earpiece and not stereo. The sound out of it is pretty decent IMO.
They did. https://play.google.com/store/apps/details?id=mobi.omegacentauri.Earpiece
Works fine for me on Marshmallow and no ads
"or" output via the earpiece:https://play.google.com/store/apps/details?id=mobi.omegacentauri.Earpiece