You won't get IM beeps if
you are in Busy mode
you are already in IM with that individual/group (i.e. have communication tab open, whether minimized or not, with that individual/group).
If neither of these is the case then I suggest, like Ann, that you re-install the viewer - it may have got it into its head that you are already in IM or busy and stuck there.
Sorry I can't help more.