You can compress an MP3 file so that it takes up less space on a hard drive using iTunes on a Mac computer. To compress an MP3, you have to convert it to lower bitrate. The bitrate is a measure of how much data is transmitted in a second. The higher the bitrate of an MP3 file, the higher the sound quality and the greater the amount of space it takes up on a hard disk.
Open iTunes. Go to the "iTunes" menu at the top of the screen and select "Preferences."
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Click the "Import Settings" button in the "General" tab.
Set the "Import Using" menu to "MP3 Encoder" if it is not already at this setting.
Change the "Setting" menu to a lower bitrate. You can select from three standard bitrate options or select "Custom" to choose from all available bitrates. Lower bitrates mean smaller files, however, lowering the bitrate too much will make the MP3 unlistenable. Click "OK" to close the "Preferences" windows.
Highlight the MP3s you want to compress in your iTunes library. Click the "Advanced" menu and select "Create MP3 Version" to create a new copy of the MP3 with the lower bitrate. Click on the original copy and press the "Delete" key to remove it. If you want to compress MP3 files not already in your iTunes library, drag them from their location on the computer to iTunes and drop them to import them with the lower bitrate.