Hello
Iam using a Mac too.Sometimes you hear sound because your pop up playing tab is still opened. If you want to make sure all your browser n its tab are closed, just right click the safari n you will see if any extra tabs are yet to be closed or look for a dot under safari icon (if there is none it means no tabs are opened if there is dot it means there are still some unclosed tabs)
Even after the above steps, still problem persist, Go to Apple Logo on left top side n click "Force Quit Option". then you can completely shut down the whole Safari for once n all ;)
- Carl