Amazon.com: Stereo Amplifier, Plug and Play Speaker Amplifier Volume Adjuster Metal Material Multi Interface for Mixer for MP3 Player : Musical Instruments
Nobsound Mini Active Volume Control Knob Volume Controller 3.5mm Audio Adjuster PC Speaker Amplifier Switcher : Electronics - Amazon.com
MP3 Normalizer for MAC OS X - Free Download - Amvidia
MP3 Volume Booster - Official app in the Microsoft Store