For a mixer/DAW/audio software,I use Audacity which is free. It’s popular since it can edit one track, multiple tracks and even record. Totally free and you can use it for professional jobs. It’s also multi-platform, so it will work on your Mac as well as it does on my PC.
More info on Audacity including links to the download are on my blog: http://videomaker.com/community/blogs/freeware/audacity/Being free, Audacity does have some quirks, such as not being able to import WAVs or export MP3s, but i show ways to get around that in the blog.
If money is no object and you’d like something better, I suggest Adobe Audition or Final Cut’s audio software Soundtrack Pro.
