What is wrong with the sound of iPhone 4s?

Everytime i captured videos on my iPhone 4s, the sound of it is very poor/low. I can't barely hear it but everything works good if I'm going to watch videos on youtube or even i played music on my iTunes application. I check the volume tools and its on high volume already. Can someone help me?

Its a known bug present in the 4S' mic. There's nothing you could do except get an external mic or install a new internal one.