Video recorded by P990i has voice missing

Recently I recorded some video clips using the camera of my SE P990i. When I played back the recorded clip, the sound was very barely audible, it was just like kind of mute. I even tried to run the recorded clip on my computer but the voice was barely audible even of full volume. I checked the camera settings and the Microphone Mute was set to off.
What could be the problem and how to overcome this. I am puzzled. Is it an inbuilt flaw of P990i or am I missing something.