Why won't my sound work?

My sound mysteriously stops working on my iPhone 4S. It will work for like 2 seconds then completely stop. It is strange, because when the phone is plugged in the sound works. When its unplugged, the ringer volume does not change, there is no bar to adjust the sound. It is a bit ridiculous and annoying, considering I have only had this phone for a few months.

This is what my phone screen looks like when I try to adjust the volume.

Anybody ever experience this? What did you do/what help did you receive?

Thanks!