1. Add a CCK file field to your content type. Make sure to include mp3 as a permitted upload extension. Make sure the description field is available.

2. Once added, change the display settings on the field to "MP3 Player (default)"

3. Upload your mp3 file. Add a description. Save.

Multiple mp3s may be uploaded to a single node, but each mp3 file will have its own mp3 player instance. It's a good idea to include a description for each file.