The standard mp3 file usually has a bitrate of 128 kbps, with that quality you can enjoy music with no problems, except if you are an exigent music lover. That size is perfect for listening and storing, good quality and small size.
If you have a lot of audio files and you need more space, maybe you can use MP3 Quality Modifier, a lite program that helps you modifying the quality of your files and take the most out of your HD.
MP3 Quality Modifier allows you to change the bitrate to a range from 10 to 310 kbps. You'll be able to modify the frequency from 8000 to 48000 Hz and the audio channels.
It's very easy to use, just drag and drop the files to the conversion list and set the output settings.