Comment 34 for bug 363892

You may have had an issue with the contents of your /var/lib/alsa/asound.state file. This determines the initial volume levels of the various mixers when ALSA is initialized before Pulseaudio starts.

Try this:

cp /var/lib/alsa/asound.state ~/asound.state.beforeboot
reboot
cp /var/lib/alsa/asound.state ~/asound.state.afterboot

diff ~/asound.state.afterboot ~/asound.state.beforeboot

If your sound goes from working to non-working (or visa-versa) between the reboots then we would hope to see output from the diff command above, indicating that the saved state of your ALSA mixer levels has changed.