the best way is to set all your song at 0.0db to start with that way you have the same output on every song .easy to do just download Audacity for the web apple store etc .load the song into the app .highlight the tracks with a double click go into effects on the top row . then select normalise. click apply .then export the track in the format you want i.e mp3 wave etc and export it to the file you've just loaded , its now set at 0.0db which is where it should be ,then repeat with all you other files .and don't forget songs have low parts and loud parts in them but this won't be effected have fun