Since you can still hear the other person when using speaker phone, chances are it isn't a simple volume error. I would still check to make sure that the in-call volume is set to max so you can be certain that nothing is coming out. Also, it wouldn't hurt to quickly reinstall your devices software to make sure that the loss of in-call volume isn't due to some unknown software error. If none of that helps, the speaker in your phone is probably broken, and will need to be replaced.