I could have the vibrate off, ring on, and hear nothing. This is happening on my daughter's iPhone as well. The only indication that a call is coming in is through the vibrate/silence option (and the display). Ringtones & music can be heard when selected. I've tried the reset solution, and it didn't work.
iMac Core 2 Duo, Mac OS X (10.4.10), 2 GB M; 500 GB HD; 2.33 GHz chip