Creating MP3s with SndSampler is easy. All you do is open your sound, then choose File > Save As. In the Save As dialog, choose MP3 as your file type. That's it! If you want to set some values for commonly-used ID3 tags, such as song name and artist, click the Options button in the Save As dialog and you'll get another dialog where you can enter this information, and also choose which MP3 encoder you want to use. Note that you can also access these dialogs from the main menu bar: Sound > Tags and Edit > Preferences > External Encoder > MP3.

In order to play an MP3 you go to the main menu bar and choose Movie > Play From Disk > Choose Files. When the standard Navigation dialog comes up, choose your MP3 and click OK. After that, QuickTime will do its best to play your MP3. You'll probably see something like this:

A cool trick you'll want to know about is that you can play any number of MP3s with SndSampler simply by dragging and dropping the files onto the SndSampler icon and at the same time holding down a special key. There are three special keys available to you: the option key, which causes the files to be played in alphabetical order (usually, the order in which they are displayed in the Finder); the control key, which causes the files to be played in reverse alphabetical order; and the shift key, which causes the files to be played in random order.

What about making MP3 playlists? Well, yes, this is something you can also do with SndSampler, but you have to do some work in the Finder first. The best thing to do is store all your MP3s in one central location on your hard drive. Playlists can then be built using folders and aliases. To build a playlist, first go to the Finder, make a new folder, and name it as a playlist. Now, select all the MP3s you'd like to have in that playlist. Make aliases of them, and drag these aliases to your playlist folder.

Now, when you want to play those MP3s, just drag and drop the playlist folder onto SndSampler's icon while holding down the option key. The MP3s will play automatically, one after the other. They'll even play in the background!

Note that this will work with any kind of file that QuickTime can play: MP3s, M4As, M4Ps, QuickTime movies, WAVs, AIFFs, etc.