This one is an odd duck - I have a medium-size iTunes library of 15,500 tunes, which are a mix of MP3 and AAC files (both protected and unprotected). For most new rips of CDs, I prefer AAC, but older items and eMusic downloads are all LAME MP3 VBR.

Here is the deal: If I take a CD and rip a track as AAC, then repeat the process with MP3 (using LAME) Sound Check assigns the two files different volume adjustment values! The exact same behavior can be observed when transcoding files from AAC <-> MP3 - the MP3s are set about +3dB louder than the AACs.

Example 1: Track 1 of the new Flaming Lips CD (Yeah Yeah Yeah Song)
MP3 Sound Check: -13.1dB
AAC Sound Check: -16.2dB

Example 2: "I Turn My Camera On" by Spoon, MP3 downloaded from eMusic:
Original MP3 Sound Check: -8.2dB
Transcoded AAC Sound Check: -10.5dB

The difference is quite audible, and appears during playback on both iTunes and my iPods. Turning Sound Check OFF causes both tracks to play at identical levels.

This becomes an issue when listening to a playlist that is a mix of AAC and MP3 files. The MP3 are significantly louder than the AACs, even if from the same sources.

Any ideas in this group? Thanks.