how do I get a 16/44.1 file from a 24/48 file on a mac?
Pretty much any audio editor could do it, I'd imagine. Audacity is widely used and recommended for audio editing, esp because it is free.
I use Amadeus Pro which I find to be a quite capable and easy to use editor. Beyond a full range of audio capabilities, it allows you to edit and save in a variety of audio filetypes including FLAC, allows VST and AU plugins for expanding its capabilities, can cut full sets into individual tracks along sector boundaries and can save cuesheets, and can tag mp3's and flac files. The price for the latest 2.0 version has gone up though (I use 1.5.5), and it is now $60 iirc.
If you just want something for a one-time thing, Audacity is your best bet. If you want an audio editor to use continually, I have used Audacity, Wave Editor, and Amadeus Pro, and all of them are good. You might want to ask in the Team Mac thread to see what others are using.