Little Ghost Girl in my House?

My house was built in 1919 and my family and I have been in this house for 6 years. And in December of 2007 my sister said that she thinks theres a little girl ghost in her room. So since then I've been afraid. And just today I was home by myself no one else was around and the TV was off everything was off. And I was recording my voice on my computer then went to play it back and when I was quiet for a couple seconds on the recording I heard what souded like a little girl's voice say "Matthew" which is my name. I listening to it again and made sure I wasn't crazy. When I heard it again I grabbed the phone and immediatley ran out of the house. (I'm 14) I called my Mom and sister and told them. They both thought I was crazy. But came home and heard it to. This only happend a couple hours ago. And I cannot stop thinking about it and am petrified. my family ured me I'll be fine since we've been here 6 years and it hasn't bothered us. So what can I do to stop me from thinking of it?