[chatrobot]

I fixed it myself.
I don't know exactly how I fix it.
But to answer the questions Bryan and Roy posted.
No, I have not change anything. This computer was purcahsed from MTU and was configured by MTU so I do not expect I need to change anything in Windows when I upgrade.
But I did check the setting mentioned in the windows upgrade required for 5.0.6 for example, energy plan set to High performance (in Windows 7), no scxreen saver and no system sound to play. They are all set correctly last night when I was struggling to find out what went wrong.
However, this monring I was looking around again hoping to unsolve the mystery, I noticed that windows cannot find my energy plan anymore (the only differences I can tell from last night to this morning is that this morning I did not plug in to AC and use battery). So I reset enenry plan to High performance. I then open Hoster but still there is no sound. I happen to open sound voulmen icon and click on "mixer" link and I noticed that with Hoster open there is one setting for Hoster sound level in the mixer windows. The Hoster sounds setting is turn all the way down. So I move the sound level up and sound start playing in Hoster.

So, as I said, I don't know why the energy plan setting was lost this morning and it seems the key is to Open Hoster and check the Mixer setting for sound.

There is no effect of playing with audio drop off setting. I change it from 0 to 20 when I was struggling to diagnostic and there is no help.

Anyway, I am back to business again and thank you for your respond.

To answer Roy's questions in specific. No I don't use HDMI connection.