How can LEDs sense darkness?
A led acts a light detecting diode as well i.e you can sense the extent of darkness and brightness by a LED but it is not going to be as effective as the one's made for this purpose specifically.

http://www.instructables.com/id/LEDs-as-light-sensors/

You can make it on your own and this instructable will prove to be helpful.

And answering your second question, yes the speaker can also act as a microphone based on this concept only.