A full version app for Android, by CodeAndPlayVn.

Volume Booster is a free program that works like an amplifier to increase the volume of your speaker, music, or headphones. This app provides mute, normal, and boost as the three quick-setting modes. Music boost is a loud audio playback that can harm speakers and hearing. You must decide to raise the volume booster by less than 40% which will make you feel secure. The majority of smartphones running 4.2.1 to 4.3 do not support this app. Up to version 4.4 should be compatible. Do not use this to change the speakerphone volume while on a call, and to reduce the danger of damage to your phone, gradually increase the volume.