As'salamu 'alaykum,

Someone asked about this earlier and I saw this link on Faraz Rabbani's blog:

Audacity: The Free Cross Platform Sound Editor

This program appears to be sufficient for the needs of most people, unless you plan on making a professional studio quality production. I personally use and like Sony Sound Forge Studio , but I had to pay $70 for it, even though I see it is going for a measly $30 now (with a rebate).