Currently Being ModeratedJan 20, 2013 9:40 PM (in response to lagartija-1003)
There is no aux cable for an iPhone.
You may have plugged a cable into the headphone port.
If you are sure mute is turned off and you have double checked by using the physical volume switches on the phone then the headphone port is probably stuck on thinking something is plugged in.
You can try a backup and restore: http://support.apple.com/kb/HT1766
but I think you will be needing to make an appointment at the Apple Store soon.