First, when you say all of the audio disappeared, what exactly do you mean?
Do you mean that the audio clips are still on the timeline but that you can't hear them?
Are the waveforms on these clips suddenly flatlines?
Where are these audio (and video) files stored? Are they on the computer's C drive, an external drive or a network?
Finally, is it ALL audio -- or only audio from a camcorder's video, an MP3 file, a WAV file, etc.?
Firstly, the audio existed with the clips 2 weeks ago. The audio track shows in the timeline (still attached to the video) and the first few milliseconds of audio you can physically see and hear. From after that point its almost as if its been deleted, just flatline the entire way across.
All of the footage is stored on the C: drive in a special folder for A Level student work. All of this is for one perticular student.
Another student has imported some MP3 files from his SD card and these appear as blank. I'm telling him to bring them in as an .aiff or .wav instead.
This sounds like the program needs to re-conform the files. (It's also possible the program is in the process of conforming the audio and it hasn't finished yet.)
Have the student open the project and not do anything for a couple of minutes. Watch for a progress bar in the lower right of the interface. This progress bar will indicates that the program is conforming the audio (essentially, digesting it and converting it into a form it can work with).
If you let it finish its conforming, you should see the audio waveform fill in and the audio will work.
At least that's the general solution. I can't be more specific without knowing more information about your video (like what kind of camcorder it came from) and your computer set-up. (Windows 7, Vista or XP? 32-bit or 64-bit operating systems?)
Finally, do ensure that you've gone to Windows Update and manually downloaded even the non-critical updates and that you've gone to Apple and downloaded the latest version of Quicktime.