Un40b7000 the volume turn down by itself.

SOURCE: The volume is turning itself down all by itself. The volume on the remote won't work, but the remote is fine with the other TV controls.

It sounds like the button is stuck. Take the remote apart if you can and see if the key is stuck somewhere. This happened to me once with a rubber key type of remote. It drove me crazy.

SOURCE: Volume won't turn down

Try the setup program. When you get to the main setup screen, look for a reset all to standard setting. That may solve the problem. If it doesn't then the sound module may need to be replaced inside the TV.

SOURCE: ViewSonic N2750w 27 in Volume turns down by itself

Could be a problem with the down volume button on the tv. the board it's on ir its harness. Check them all. Make sure the volume buttons plastic actuator moves freely and is not stuck.

If these parts are all good, you have a problem on the main board. You may have to trace the control lines as far as you can and check any capacitors along the way. If you have an oscilliscope you can test the lines to see if the up volume is working correctly. If there is noise on it, you might find a bad capacitor if you can trace it back far enough. Schematic helps here.

Of course, make sure nothing is shining on the sensor window on the TV. You might want to scope that line as well (IR sensor line).

If none of that helps, get hold of Viewsonic and find your closest service provider. You can order a main board from them.

Of course, you could try just using the audio out jacks to an amplifier or a surround sound amplifer. Then you would use the remotecontrol for the surround sound system to control your volume.
Bet you were going to buy one anyway right?