I had a similar issue. I could hear the speakers pop during boot up and all the media players functioned like I had sound, just no speaker response.
I used alsaconf. Run it from the prompt and it will unmute any muted channels (PC speaker, CD, etc) also Im assuming you are running something from a GUI. (Gnome / KDE). You can test the sound at the prompt by using the play command. basically
play FILENAME.MP3 (Of course where FILENAME is your sound file)
See if that helps. If not , you might have a sound driver issue and I'm unfortunately not a person that can help there.