How big is your video file? How many minutes? Remember that Photoshop is primarily for photo-editing and a bit of DSLR Video editing. If audio is very critical to your project, consider using Premiere Pro or something.
If your file is a considerably long video, you will face issues with Audio and this is a known issue.
Otherwise, take a look at this Lynda.com tutorial on Youtube to see how to enable/ edit audio in PS CS6 - http://www.youtube.com/watch?v=h51FJbGB8dg
It is what you call, a considerably long video...
2 GB's to be exact.
Thanks for your reply and solutions.
What is the duration in minutes? There have been known issues with video longer than a couple of minutes.
The duration is 1h 57'!
No wonder! This is a known issue for videos longer than a couple of minutes.
Tough luck! But, try out Premiere Pro for editing your videos that are this long.