browser mute on sleep

How can I get my browser to mute when I close the lid on my macbook?

When im listening to music on soundcloud or youtube... and i close the lid to pack my laptop onto my bag my browser doesnt mute/stop playing music.

When I open it up in school or else where.. the music resumes and i always got to lower the volume/mute it so people dont get disturbed.

help me please.