Go to the address www.audacity.sourceforge.net and download Audacity according to your operating system. Audacity is a free, fully functional music and recording studio capable of handling all types of audio files. Install it somewhere with easy access. Locate the MP3s you wish to adjust and save them in a folder on your desktop.
Open Audacity. In the File menu, choose "Import -> Audio." This will bring you to a file browser to locate MP3s. Choose a file you want to adjust and select "Open." The sound file will be loaded as a wave form in Audacity.
Press "Play" and observe the sound levels. While the song is playing, adjust the track volume in the wave on the left. There will be a "+" and "-" button above "L" and "R." Raise the volume to desired level and stop play. Go to File and select "Export." This will allow you to tag the track and rename anything you need to. Select "OK" and choose to save the file as an MP3. You can either replace the old file with the newly adjusted one or save it for future use. Repeat Steps 2 and 3 for multiple tracks.