Assuming you have the Sony PC101E (and not the Sony DVD101E), I own one of these cameras and have never come across a problem with audio recording from the inbuilt camera mic. Try using a handheld mic for a test recording to narrow down what the problem might be.
Sorry I can't shed much more light on it than Tim has, but would just like to reiterate his second point as handycam tape heads can become worn very easily, especially if you leave it on standby a lot.
As far as the manual goes,
DCR-PC101E (DCRPC101E) : Camcorders : Manual : Sony : United Kingdom will give you the manual á la PDF.
Take it easy, Rgb