A lot of the system volumes were put back to default. I only realized this when I downloaded the "Audio Manager Widget". Then I found that I apparently missed a couple volume settings.
also It might be due to the In-Call volume. The only time you can adjust the In-Call volume is when you're in a call (duh!). So, I called my voicemail and put it on speakerphone just left it all the way up and now I can hear the dialpad tones again!
also I think it was swapping from vibrate to full volume when I received an SMS (or was doing multiple things at once).
All sound stopped working on the device. It's an Dream from Optus.
try a quick reboot.. that shud fix all the problems though

if all the above moethod fails... its time to call the manufacturer....

good luck