Does "Sound Check" actually work?

I have songs I've downloaded from my personal library of CDs, from the Apple Music Store, and from eMusic. When I loaded these songs into iTunes, I checked the "Sound Check" box under Playback Preferences, but the songs play at such different levels I have to constantly adjust my speaker output level.

Does the Sound Check preference for iTunes actually work? If so, what am I doing wrong? (And if not, why doesn't it work?)

I realize I can Get Info for each song in iTunes and adjust the volume level, but this seems like a royal waste of time. Is there a way to adjust the levels for all songs at once?

Thanks in advance,

BILL