A free app for Android, by ALPHA LAB.

The AUDIATOR BOOSTER app is an mp3 volume booster for Android phones.

The application is based on the FFmpeg library, which is used for audio processing and volume boosting.

It has a simple and intuitive user interface, allowing you to easily search and download your favorite music, audiobooks, and ringtones. You can also set the output destination with a custom frequency, bitrate, channel mono, channel stereo, levels of amplification, and the amplification mode allowing your headphones and speaker to produce quality and loud sound. 

You can download the app for free and try it out. You can also get the pro version if you like it.