play a preloaded track on the page where its says: Thanyou for logging in.... which lasts a few seconds? Have a beep or a ding sound? or maybe a woosh sound.. which lasts 2 seconds... should be possible.
It is possible, I just don't know where the template or code for the log out/in script is stored, I would put sound from an online game "Battlefield" saying "Welcome back soldier" and "Are you going awol soldier?".