After creating a project in Garage Band and sharing it to my iTunes library the output volume is very low.
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I was having a problem with really low audio levels in iTunes (I've just installed Mavericks). The original mp3 files played fine in any other app.
Then, in iTunes, I clicked on the 'Mirror' icon (orange on my system) beside the iTunes audio control in the upper left hand side of the screen. Problem solved, the audio settings here were set at about 25%, when I dragged the 'Multiple' slider to 100% I had full control of the sound level again. Also, I noted in individual tracks stored in iTunes you can click on that track, then click on the 'Get Info', then select 'Options' and individual audio controls for that specific track are available.
Hope this might help.