There used to be a plugin that would minuter the hb and game client and would send you a message if you received a whisper. But if you are to where you can hear your upstairs PC from downstairs than try turning all wow sounds off, download the addon "WIM" and configuration it to play a sound when wow sound is disabled.
This is how I am set up so I hear when I get a whisper
The BuddyCenter plugin lets you setup Notifo or Prowl. I'm using Notifo together
witgh my galaxy s. Get a status update every 30 minutes, also you can set it up
to get a msg when things are being said in guild/raid/party/Bg chat etc.