To edit the audio you need to make changes in the your presentation, then update the presentation and again convert to video. Please follow the steps below to download, edit and replace the presentation in your account:-
1) Sign in to your authorSTREAM account.
2) Go to My Stuff page: http://www.authorstream.com/MyUploaded-Presentations/All .
3) Under the download menu -> click the PowerPoint option to download your presentation.
4) After downloading the presentation to your computer, you can edit your presentation and then replace the PowerPoint file. See more details to replace a PowerPoint :- http://www.authorstream.com/help/74-replace-presentation/
After your presentation is replaced, you can again convert PowerPoint to video. Please verify the output of the presentations at authorSTREAM before creating video. Video output is almost similar to authorSTREAM output (Presentation played in player).