While Snag It is wonderful for screen captures (used 25-100 times a day here) it simply does not have a project timeline that would allow you to do what you want here. It's big brother Camtasia is perfectly suited for this and I've done it a few times. How many slides and length of audio?
Record PowerPoint Share more than just the slides from a presentation, use the Camtasia Studio PowerPoint Add-In to record and share PowerPoint presentations for attendees to watch again or for people unable to attend the presentation. With the PowerPoint Add-In, you can: Quickly record and produce a PowerPoint presentation as an interactive video to appear on a webpage. Include audio or camera video to include as a Picture-in-Picture recording. Save the PowerPoint recording as a Camtasia Studio project file (CAMPROJ) to edit or produce into a variety of file formats. Automatically add markers to the recording file for each slide in the presentation. If you include a table of contents in the final video, the markers appear as table of contents entries
You could open the PowerPoint slide presentation, and play the audio in a media player, and then record all of that with Snagit.
You will just need to be careful of keeping everything in sync.
While the trial of Camtasia will work, the trial is to be used only for evaluation purposes before you choose to buy or not. So, the content you create with the trial cannot be used for public consumption, training, posting on the web, etc.
However, if this one video is something very important to you or your company, you are able to buy a 30 day license from the Steam store for just $14.99: http://store.steampowered.com/app/253050/
Content created with the 30 day license can be used for all your editing and publishing needs.
I hope this helps!