I just run my mp3's that lack a full PUNCH through the Sony(previously Sonic Foundry) sound editor...Sound Forge.
A simple run through the wavehammer pre-set will re-master the file, and it sounds sonically great.
The version they sell now has noise reduction,which will take out any form of hiss or trash.
Just an idea for people with a lot of live recordings from various tapes,sources and compressed formats.
PEACE