There are two most likely issues that could be causing your speakers not to work:
(1)Bad headphones/speakers
It's unlikely your headphones or speakers are bad, but it's worthwhile to eliminate these as the source of your problem at the beginning. Try your Nokia Lumia 635 with another set of headphones or speakers just to make sure that the problem is with the Nokia Lumia 635.
(2)Bad audio jack
The most likely cause of audio output problems on Nokia Lumia 635 is a bad audio-out jack. You can purchase a new headphone jack and replace it.