Pete, whilst I don't know much about the specifics of Sound Forge, I do know a bit about sound editing (Cool EditPro). Quite often, sound files from the internet have either been recorded (ripped) badly by someone or more likely have been too heavily compressed (either to save on download times or to encourage you to purchase a better quality version. All mp3's, by their very nature, are compressed - i.e sound information has been thrown away to reduce the file size. Usually this loss of information is barely audible to the human ear. Try opening your MP3 in Winamp (
http://www.winamp.com ) . Winamp will tell you the quality of the file in kbps in the player window. If it's less than 128kbps then it means that the file itself is of an inherantly poor quality - and there is not very much you can do about it.
If it's muffled (usually a sign of heavy compression) you could try adding some treble if Sound Forge has a graphic equalizer function. If the music track is commercially available you could try buying the CD, or if you got it from Kazaa try downloading the song again. Always download the file with the largest size, this usually means it's of the highest quality. I really do recommend Cool Edit Pro or Cool Edit 2000 - from
http://www.syntrillium.com They really have got a massive array of controls for reducing noise and faults with sound. Hope this of some help.