That's close to all I know to try .
In Device Manager did Azalia show as the sound card ?
I had thought of checking the temperature of your laptop but you said when you tried just video it went for several hours without a freeze . So from that it shouldn't be overheating .
If you want to stress test your laptop just a bit.
Use a software that encodes video like AVStoDVD setup to use HCEnc 2 pass .
Choose a large video file Like an .mpg or the folder from a ripped DVD even if it is encoding back to a DVD . If you watch to temps you should notice an increase from normal probably 15 to 25 degrees higher . If heat is the problem this should cause a freeze.
If heat is the problem that points some to a failing sound card.
That is why it lasts 20 minutes or so . Processing the audio data OK until this creates enough heat.
If that is the case it would mean replacing the motherboard .
I honestly don't know if there is an external sound card for a laptop.
Maybe someone will post if there is. I may take a look just so I know.