Making incoming messages bold would help with usability.

I have tested it on several people over 30, all of whom could not hear it. Try making him tell you when the buzz is on. It is possible that he imagines that sound. People hear what they want to hear, and finding that your hearing is going it no very nice....

Also, what speaksrs are you using? As I said before, there have been reports that some speakers will not play the sound correctly, resulting in no one or everyone being able to hear it.