Exploring the files in a MobileSync backup
Here's the DIY method:

Navigate to the following directory:

~/Library/Application Support/MobileSync/Backup/

You'll get a cryptic list of folders.  Inside each folder will be a large number of files with cryptic names.

Open these files in Quicktime Player.  

There may be thousands of them but if you sort by size you should get video, audio and photos to float to the top.

Alternatively you could try one of the applications that is meant to do this such as iPhone Backup Extractor (There is a free and a paid version I have not tried either myself).