Different tracks may have been mastered at different levels. It happens sometimes, but there isn't anything that you or OnSong can do to prevent this. However, you can utilize a setting that Apple now has called Sound Check. When that setting is turned on, it will go through your music and neutralize everything to be on the same level. This can be turned on in your device's Settings app > Music > Sound Check. Turn that on and you should be set to go from there.