APPLE MAKES NEEDED CHANGE TO SOUND CHECK IN ITUNES

Sound Check is a very convenient feature in iTunes that normalizes the playback volume across all songs, so you will not have a song that was brickwall mastered blasting out 5 times louder than a song that was mastered with more dynamics intact. This is extremely useful if you have created playlists that include songs from different eras and genres that have widely varying volumes, or if you are just playing your library in shuffle mode.
Until now, a problem with Sound Check occurred if you were listening to a complete album in album mode. Artists/producers often purposely mix some songs on an album at lower volumes to create an emotional effect through dynamics, or volume differences (that's why dynamics are referred to as "expression" in music, the dynamics express emotion). And if the album includes both electric and acoustic songs, the acoustic songs should be perceptually lower in volume or they will diminish the  electric tracks. But with Sound Check turned on, it adjusted the level of the all the songs to a common volume, destroying the effect the artist/producer intended.
The good news is in an iTunes update sometime around the end of July, Apple made an unannounced change to Sound Check. Now, if you are playing iTunes in album mode with Sound Check, the original volume differences between tracks on the album will remain intact, with the overall volume of the album being brought down so the loudest song is at the AES recommended volume for streaming.
It's not perfect yet; it doesn't seem to read the volume of the loudest song properly, so if you switch from song or playlist mode - where Sound Check is adjusting the volume of each song - to album mode, the album mode is louder than song/playlist mode. Not up to full volume as without Sound Check, but just enough to be a little annoying and require a volume adjustment. But that's a very minor detail that should be ironed out soon, the point is the dynamic flow of the album is preserved without having to go into preferences and turning Sound Check off.
If you're not familiar with Sound Check,  open iTunes, go to the top left "iTunes" pulldown menu and select "Preferences". At the top of the preferences window select the "Playback" tab. You'll see a checkbox to turn Sound Check on. Also, be sure "Sound Enhancer" is turned off. And, if you want to hear the album exactly as intended, turn "Crossfade Songs" off as well.