For example, I had a concert that was truncated by commercials, and I took the separate MP3's and edited into one seamless concert with no gaps between tracks. Sound Forge is readily available through http://www.sonymediasoftware.com/products/soundforgefamily.asp.
Also, when you import the whole shebang into, say, Nero, for burning, you can increase the bitrate which tends to boost the overall volume (I've found), but both Sound Forge and Goldwave give you the option of adjusting the recording volume.
Goldwave is also an awesome program for converting your old LP's and tapes to digital format. There's some pretty robust tutorials on the company's website.
Hope this helps. Oh, also, the "professional" choice would be Pro Tools which is a pretty advanced mixing program, but it's freeware which is nice (at least it was a few years ago) and tends to be a system hog.