If you want to play audio from a folder, you need to open it in the audio player. 1by1 Directory Player will do that for you.

When you open it, you will see the main screen. The first thing you need to do is to select your audio folder.

Select a folder. You can open it using the built-in file manager or by selecting it from the list.

When you are done playing, you can exit the app.

This is available on Google Play Store.