Why is my iPhone sometimes really quiet, then sometimes really loud?

[mr.stinki]

I seem to have this problem a lot. All of sudden it's really loud, then i sync it, and it gets soft, then after a while it'll get louder again.

Why does it do this?

OH, BTW, this is only with system sounds (like unlocking, keyboard clicks, mew mail) not music or ringers...

 

[hotsauce]

Wirelessly posted (Apple Communication Device: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420.1 (KHTML, like Gecko) Version/3.0 Mobile/4A102 Safari/419.3)

Watch where you place your hands. The speaker is the lower left one. It's a common habit to block that speaker with your finger when typing.