tAdding an audio file (mp3) to a post is relatively straightforward. Make sure you have the mp3 on your desktop. Once you do, click the Upload/Insert media icon (pictured to the right) and drag the audio file from your desktop onto the window. Once you have done this the mp3 file will upload automatically.
After that re-title the file with an appropriate title, and then be sure to click the Audio Player button and then insert the file into the post.
That should be all.