The speaker/microphone on my iPhone doesn't work properly with video recordings but works perfectly with 'Voice Memo's'

Hi, I was just wondering if anyone has had this problem and is able to help me with it - When I do a video recording on my IPhone 4S and play it back, the sound is low and distorted. I have done several voice recordings in 'Voice Memo's' to test the microphone and speaker but they both seem to be functioning properly as it worked perfectly. The sound also works perfectly with everything else including music, ringtones, texts etc. - If anyone has had this problem and/or knows how to solve it, it would help greatly. Thanks Very Much.