internal speakers do not work on the I iPhone 4s

Close All Open Apps... Perform a Reset... Try again...

Reset ( No Data will be Lost )

Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.

http://support.apple.com/kb/ht1430

If no joy...

Connect to iTunes on the computer you usually Sync with and “ Restore “...

http://support.apple.com/kb/HT1414

This is most likely caused by dirt tripping a sensor in the headphone jack.

This video will show you how fix that.

http://www.luckow.com/iphone/

I recommend using rubbing alcohol instead of windex.

I had the same problem, this worked perfectly for me. Hope it helps!