If you're using Garageband to create your tracks, just follow the directions below to export them as 320 kbps MP3 files.
First save your project. Click on File and click Save. Next, click on Share and then Export Song to Disk. This will bring up a window where you can change the name, location, compression format and audio settings you want to set for your exported MP3. In this section, make sure you select MP3 and use the drop-down menu to select 320 kbps.
When you are ready, click the Export button. This will bring up a window box with Bouncing… and then Converting to MP3… progress bars. Once those two progress bars close, your file has been saved and is ready for you to upload :)