My laptop (a Toshiba Sattelite Pro L40) has working sound now! It started woking after I added the following two lines to the file /etc/modprobe.d/alsa-base

options snd-card-0 index=0 model=3stack options snd-hda-intel index=0 model=3stack

The problem was that the audio was working when I first started the laptop, but it would stop when I used the volume mixer. Changing the volume did something that made the audio stop working.