Probably the most lightweight audio player for Android. Download size less than 1MB.

Works only with local audio files (that are stored on the device).

Features:
- audio formats: mp3, m4a, wav, some more depending on the device
- simple old-school GUI
- open file or multiple files to play
- can also be started via audio file Intent.ACTION_VIEW or Intent.ACTION_SEND
- play in the background with screen OFF
- seekbar, rewind and forward buttons +/-1 min., +/-10 sec.
- real-time audio processing, no messing with the system volume
- adjustable gain, dynamic / constant, max. 60dB

!!! LOUD SOUND WARNING !!!
Loud sound and / or prolonged exposure to loud sound can cause permanent damage to your ears, headphones and / or device speaker. Set your device audio volume to a safe level that is not too loud. In the app settings there are two options: dynamic gain and constant gain. If you choose constant gain and hear distorted sound, make sure to reduce the amplification level (dB) using the slider. When using headphones, you may be unable to notice surrounding sounds. Make sure this app does not distract you from important sounds around you.