What is the correct way to get Final Cut Pro X to create 320 kpbs audio?

I want to export a video that's 1920 x 1080 and is 60 fps. The problem is that the audio track I used for it is the highest mp3 data rate allowed. Final Cut Pro only exports it's HD movies with an audio quality less than that. Please tell me how to export with the best quality in mind, but unfortunately I can't use compressor.