Sound editing

Audio Editing

Low End

Audacity | Free, easy to use and good results.
http://audacity.sourceforge.net/
Garageband | Part of Apple’s iLife software bundle. Easy to use and good results.
http://www.apple.com/uk/ilife/garageband/

Mid End

Apple Logic Pro | ‘Whether you’re a professional or play in your garage, Logic Pro 9 gives you everything you need to write, record, edit and mix music.’
http://www.apple.com/uk/logicpro/
Serif | See comments about Serif Design Suite in ‘Video Editing’.
Sony Acid Pro | See comments about Sony Vegas Movie Studio Platinum 10 or 11 above.

High End

Adobe Audition | Part of Adobe’s Creative Suite. Industry standard.
http://www.adobe.com/uk/products/audition.html