Here is a
list of links to various audio editors .
I researched this last year as i wanted to get the Japanese Bible and 1858 Great Controversy from cassette to CD. I finally went with Goldwave. It is very powerful.
For professional quality of software tho, Cool Edit Pro is probably the best. Soundforge is rated pretty high too.
Goldwave is shareware, and after i think 30 days or so, you can do 150 actions before it starts incessantly reminding you to pay up. I've written the author to find out how to pay easily from Japan, but received no response, so i usually close the program after 150 actions and re-open it.
If you want to hear a bit of what i did, look
at this page near the top for Great Controversy audio. I compressed it down quite a bit for the mp3 version on the internet, but the original WAV version is very nice