Sound Check in iTunes
I’ve found that unchecking the Sound Check box definitely does increase volume on some songs; however, on others, it reduces volume. It depends on the sorts of recordings you have as to what actually happens in each song. If most of your music is really loud, using sound check will, generally, reduce the volume. The opposite is true if your music is typically soft. You’ll get MORE volume in that case. However, it seems that the 4th generation iPod doesn’t have as much volume as my 3rd generation one, even with sound check unchecked and volume limit set to maximum. So you’ll either have to just accept this, or find a set of earphones that play louder.