Vista - No sound from laptop speakers-Vista problem source

My daughter's Compaq Presario F700 laptop has an intermittent no sound
issue. It works through the laptop's speakers for a while, stops working for
a while, and then starts working again. She is not aware of any specific
activity that causes the symptoms. Running Vista SP2. The issue was
happening before and after the service pack update (I do not believe it is
related).
I have done due diligence to troubleshoot the usual suspects: the mute is
not engaged. The sound level is turned up. The device says it is working in
device manager. I have uninstalled and reinstalled the sound device
(Conexant HD-SmartAudio 221). Updated to the most recent driver. I have used
an Unbuntu disk to check the sound from the speakers outside of Vista - they
work in Unbuntu. We also get sound from the headphone jack. Just not in
Vista. There is no error log regarding a sound problem.
A while back she said she got a message that said something like
"enhancements for sound device are causing conflicts - do you want to
disable enhancements?" she answered yes. The sound worked after that for a
time. Just 2 days ago it stopped working again. No new message has come up.
I obviously need advanced diagnostic instructions. Any help or links?