^{Update by Mega_Spud:}
Summary:
When disconnected from a server (LAN, for example), you can continue to hear sounds from the world, even though you are disconnected from it.

Steps to reproduce when playing over LAN:

1. Join LAN map as a client
2. Stand near enough to a nether portal to hear the sound it makes
3. Get the LAN host to quit the map

Observed Results:
Notice during the Disconnected From Server screen that you can hear the nether portal sound continuing to loop plus any nearby mob sounds

Expected Results:
Any sounds should stop.

Screenshots/Videos attached: Yes

Notes:

Original Description:
Disconnect from a multiplayer world due to timeout
"Disconnected from server"-Screen plays sounds.