...try
mp3 direct cut for cutting and splitting, take a look at the webpage
here!
...for adjusting the volume you could use
mp3gain. Here's the
link!
...if you really need effects than you have to use a sound editor and reencode (:spank: "Write 100 times I will not reencode any mp3's"), but be aware of the immense quality degradation, if you reencode mp3's.
EDIT: I need to type quicker the next time, damn!