A free program for Android, by Vintage Audio Tools.

BASS Booster Pro is an application that boosts your sound quality. BASS Booster Pro can help to increase the volume and bass levels.

It is very useful to help to increase the sound quality of a phone or audio player. This app can give you a lot of options to increase the volume and bass levels. To improve your sound quality, use headphones or external speakers. Enjoy listening to your favorite songs with this simple and easy-to-use bass booster app. 

This is a free-to-use application. However, the app uses revenue-generating ads for its development and maintenance.