Using software like iTunes you can alter the bitrate of a song ..which changes the size proportionally, but the quality will drop also. I suspect you are trying to free up disk space. I have done this myself, but realized that a better idea was to install a second hard disk ..(you could use an external one) and keep your files good (or you can burn them to data disk or audio CD). If your files are 128 bt rate, you should not try to make them smaller. The quality .. not good at 128 .. will get rotten.

I hope this helps.. let us know if want to know more.

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