From time to time, users report hearing sounds when a new window or tab is opened. I'm not sure anyone on Mac has reported back on what causes it.

Here is a recent thread which refers to a tune: How do I keep that stupid tune from playing every time I open a new tab? Could you try the various troubleshooting steps suggested there?

Go to Plug-ins. Click Never Activate for Quicktime. Worked for me.

I'm on a Windows 7 PC, but had the same problem since re-setting FF following a nasty "Ads by Info" infection. None of the suggested solutions here or on other forums worked. Finally found a setting for the NoScript plugin (a must, IMO): under the Notifications tab, unticked "Audio feedback when scripts are blocked." That was it - NoScript was blocking scripts on new tabs, causing the notification. Also happened when first clicking on an unviewed tab. A-OK now!