This is a standard Android Music Player with the following changes:
1. Volume control is set at a fraction of the current media volume. For example, CoPlayer can play at 50% of the current media volume. This is handy for playing background music at lower volume while listening to a foreground media at higher volume.
2. Will not mute other apps.
3. Will not be muted by other apps.
To use CoPlayer:
1. Start your foreground app, Set its media volume.
2. In CoPlayer find and play some audio for your background sound. Set its media volume (it's always at a fraction of your foreground app media volume).
Music CoPlayer has also been tested to work as specified above with my app "Meditation Creator" (https://play.google.com/store/apps/details?id=com.meditationcreator).