Ok, I figured something out. But I still need your help. The doorbell sound happens when I receive an email at my Yahoo mail account. I know this because my iPhone alerted me at the same second as my Mac. However, this is odd because I don't have my Yahoo mail account linked to the Mail app or Thunderbird or anything else that I know of. Anyone know why this is happening?
Do you use any chat apps? It might be a notice that comes through that.. I know in my Yahoo MEssenger I have to go in and disable the sound alerts for New Mail, etc...