To upload audio, click "sound." Then click "upload voiceover" or "upload music" to upload an audio file. The editor currently supports MP3 files under 5MB.
In each wideo, there are two audio slots so that you to have a voiceover and a song playing at the same time. You can adjust the volume for each of these files independently of the other. For the moment you can only upload two tracks that play for the duration of your wideo.