Audition, Samplitude, and Audacity will all provide sufficient functionality in the three scenarios about which you ask, though Audacity's the weak link (but on the other hand, it's free). I'm sure ProTools and SoundForge fall in line with Audition and Samplitude in applicability to your scenarios. After using Audition, WaveLab, Audacity, and Samplitude, I've finally settled on Samplitude since I like it's interface and object-oriented workflow best. The only one to decide, I think: try 'em all, see which one you like.