Hi,

one of the best FREEWARE used to edit audio data (including mp3 - with the addition of the free LAME encoder/decoder) is Audacity.

It's open source, available for different platforms (win, mac, linux) and works fine.
In their site there are dedicated downloading pages, including the above mentioned Lame encoder/decoder (+ instructions how to install), plug-ins, etc.

Regarding editing a mp3: there are some editing possible without decoding/editing/encoding procedures: two I knows are:
- removing silence from beginning and ending of the song (but this is removing packets of data, so you cannot be "sample" precise, you can work with approximation).
- changing the overall volume (this is made adding/subtracting values from the encoded data).

Anyway most of editing procedures requires decoding to linear format (wav, aiff), editing and encoding (that means loosing some quality).

Hope this help.

Regards.