maybe you can take apart the laptop and use a multimeter to verify if any electricity is going to the speakers.
sometimes when speakers are blown they seize up and wont play any sounds, even though there is electricity going to them.
it isnt an uncommon thing for people to blow their laptop speakers.
maybe when you take it apart you find that somehow the connector magically became unplugged.
you'd be lucky to find such a thing.. but if you dont get in there to look, somebody else will (and it might cost some money).
just think.. if your laptop speakers refuse to work for whatever hardware reason, a new soundcard probably wont play audio through those laptop speakers anyways.
you usually have to wear headphones or get some amplified speakers.