iTunes 4: About iTunes Store Song Bitrate
Songs purchased and downloaded from the iTunes Store are AAC Protected files and have a bitrate of 128 kilobits per second (kbit/s).
The purchased song should sound as good as or better than a 160 kbit/s MP3 file. Because the bit rate is lower, though, the AAC file takes less disk space than the MP3 file.
|93013||iTunes 4: AAC and MP3 Codecs Compared|
|93012||iTunes 4: About Advanced Audio Coding (AAC)|