How does audio work on Android?
Android uses separate audio streams for playing music, alarms, notifications, the incoming call ringer, system sounds, in-call volume, and DTMF tones. This allows users to control the volume of each stream independently. By default, pressing the volume control modifies the volume of the active audio stream.
How do I increase the volume of an audio file on Android?
Go to Settings > Audio and check the box labeled Volume boost.