Once you have your sound profile set to normal, reselect your profiles, select advanced, select normal, select phone and verify that you have your out of holster set to tone, that you have the aproprate ring tone set and that you do not have the volume set to mute.
If you have your ringtones or sounds on an SDcard, problems like the one you described will/may occur because when you plug the BB into a USB cable, the BB goes into mas storage mode. As you've probably seen, in this mode, the SDcard becomes inaccessible to the BB, but appears on your computer as a drive letter.
In my case, if any event is triggered that needs a sound file from the SDcard while the BB is plugged into my USB cable, the BB gets confused. Usually a default sound will be heard, but, in rare cases, the system gets confused and even after you unplug the USB cable, the sound doesn't restore until after a full battery pull.
Fortunately it's not too frequent. Only bad thing is when my morning alarm (using Myalrm) is affected.