how can I get my downloads as mp3 instead of aac?
You can't from the iTunes Store, but can manually convert them afterwards. Some other stores sell MP3 music.
If you're referring to songs from CDs, change the default encoder in the iTunes preferences to MP3.
The iTunes Store delivers music content in 256k AAC format (or 128k Protected AAC if you redownload old enough purchases that haven't been upgraded). If you need your music purchases to be in MP3 then you might be better off using the Amazon store.
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