When you drag the HTML5 Audio stack out to the page, you will see the above stack appear on your page.
Now you can set your link for your mp3 file by clicking on the Set Link button in the stack settings.
If you want your audio to work in other browsers, you should also include OGG Audio and WAV Audio files. You can use your mp3 file to convert the files to these formats. We suggest you use the Free Miro Video Converter (this will work with audio too - MAS Download or Direct Download) to convert your files. Once the files are converted, then click on the Set Link buttons for the files you want to add.
You can also use Rapidweaver's Resources (see the HTML5 Audio - Using Rapidweaver's Resources Area for Audio File FAQ to learn how to do this.) to link to your audio file if you do not have it already on your server.
Now preview and enjoy! For an overview of the remaining settings please refer to the HTML5 Audio Settings section.