In my experience over many years of using iTunes as my Music Manager, Sound Check does not work well in equalizing the volume between songs from various sources.
I have gradually adopted the practice of manually changing the volume of individual tracks. Each tune has a volume slider, found by doing a Get Info and selecting the Options tab. This can be a tedious procedure, especially if your music library is extensive, but it's the only dependable method I've found.
I have seen several downloadable utilities that claim to solve this problem, but they all charge a fee for continued use, although some have free test periods.