I'm just guessing, but under "My Settings" there is an option to enable email notification when it is your turn to move. I believe my computer makes a sound when it receives an email. So if that option is enabled you would probably get the sound. Of course, you'd need to look at the email to see if that was the reason. Since I am trying to get rid of the sound, I haven't enabled it.
Also, I once had an app called "RHP at Work" or something like that. It was just another version of RHP (there seem to be several versions). That one put a notification in the upper right corner of whatever screen was active whenever it became your turn to move. I can't remember for sure, but I think it also triggered a bell of some kind. I didn't want the sound, obviously, so i deleted it and went back to the basic RHP. I'm sure you could find it with Google under RHP (probably spelled out) At Work.
I've wondered if that notification sound somehow never got deleted when I got rid of that version, but I don't know how to find out.