Compressing it depends on the current quality of the track. If it is 192 or above compression can take place, if its less then its not going to benefit really.
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To lower the quality do the following:
Go to iTunes > Preferences (or cmd+,),
Click on the 'Import Settings' button,
Change 'Import Using' to MP3,
Change the setting to 128 or 160 - or go custom and use your own rules.
Click OK on both windows.
Select the track you want to compress, right-click (or control+click) and select 'Create MP3 version'. This will create a duplicate track with the options you set in the 'Import Settings' tab...
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If you want to trim the song, right-click (or control+click) and select 'Get Info', go to the Options tab, and you will find 'Start Time' and 'Stop Time' boxes, simply enter the time you would like to trim to. Then hit OK.
After that, simply right-click (or control-click) again and select 'Create MP3' version and this will create a duplicate with the trimmed times.
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If you are happy with the changes you can then delete the original MP3 or back it up then delete it.
Hope that helps and you didn't end up more confused after reading that...
