I can't hear audio when recording video on my 4GS?

I tried recording a video on my iPhone, and noticed that when I hit the record button, it didn't make a sound like it did on my other friends' iPhones. I thought this was strange, and sure enough, when I played the video I had just recorded, there was no sound. I played a ringtown, opened an app with sound, and played a vido from Youtube and it all works fine with audio. I tried restarting my phone, I went into the settings and messed with the sound, turned it on and off mute (several times), and made sure the ringer was turned all the way up. I can hear the audio on my videos when I plug in headphones, so I know it's recording the audio, but when I try to play it back on my phone speakers there is no sound. What gives?