Sound volume varies from channel to channel.

go to the Menu and you should find it.

Check your aerial connection then retune again.
It could be that set has picked some very weak channels, for those you mention. So make certain you select the option for best signal when retuning.

Check the orientation of the antenna. Try to rotate it, and see whether the signal strength varies. If the signal strength is too weak, digital channels will show many problems. Try a signal booster. Contact any service technician. http://electronicshelponline.blogspot.com/

If you are receiving a "No Signal" error message, verify that:

1. All of the cables physically connected to the TV and the input device are secure.
2. The input device is turned on.

Your TV needs to scan for channels before it can show programming from an antenna, from cable TV when the cable comes directly from the wall (no cable box), or from a cable or satellite box connected to the TV with a coaxial (RF) cable. Do not scan for channels if you have a cable or satellite box connected using AV (Composite), Component, or HDMI cables. The box selects the channel, not the tuner inside your television. If you are not using the "TV" input, press the INPUT button on your remote until the correct input mode is selected.
To perform a channel scan using the coaxial input:

1. Press the INPUT button on your remote to view your current input source. The TV (coaxial cable) input must be selected. This may appear as "TV," "ANT," or the channel and call letters of a station.
2. If the TV input is not selected, press INPUT until the TV input mode is selected.
3. Open the on-screen menu and select TV or Tuner option. (This varies by model.)
4. Highlight Tuner Mode and select the correct option:
   • Antenna-over-the air signals from an antenna
   • Cable-direct from the wall cable TV, or a cable or satellite box connected to the TV with coaxial cable
5. Select Auto Search or Auto Channel Scan. (This varies by model.)
6. When the search reaches 100% complete, exit to the menu.
7. Check the available channels:
   • Antenna or direct-from-wall cable-Press the Channel Up and Channel Down buttons on your VIZIO TV remote to view the available channels.
   • Cable or satellite box- Will return only one channel. Use the cable or satellite remote's Channel Up and Channel Down buttons.

If this does not resolve the issue, power cycle the TV and then re-run the channel scan:

1. Turn off the TV.
2. Unplug the TV from the electrical outlet.
3. While the TV is unplugged from the wall, press and hold the Power button on the TV for 30 seconds.
4. Release the power button.
5. Plug the TV back into the electrical outlet.
6. Turn on the TV.

Note: There is also an option to reset the TV to the factory default settings. This option is in either the SETUP or SYSTEM section of the on-screen menu. The option may be "Reset all settings", "System Reset," or "Reset & Admin." Consult your TV's user manual for the location and steps for resetting your model.

1. Remove the coaxial cable from the DTV/TV connector on the back of your television and perform a channel scan to clear the existing channels. See the procedure above. This should not find any channels.
2. Re-attach the coaxial cable to the DTV/TV connector and perform a new channel scan.

This video shows troubleshooting steps to use when a "No signal" message appears.

The TV reads the PC via the HDMI out from the computer. You must make a direct connection from the PC to the HDMI input on the TV with a proper HDMI cable. Once that is established, you need to be certain what HDMI Channel is represented on the TV, HDMI #1, 2 or 3? Etc. Best you turn on the PC AFTER all connections are made and the TV is in full operation and the channel HDMI 1,2,or 3 etc. are on. Now turn on the PC and see if you get a connection. You may have a picture if you are streaming a video, and NO sound. If so, reset the Audio by clicking on the speaker icon at the lower right of your computer screen, and in the Volume Control Options select the output you wish to use.

I think you have an off centered voice coil of your speaker..

make sure set not set to sap mode as this can blank audio on certain channels

Hello Blwas5 hi, If some channels have sound and others none, its either the TV has poor reception or, its the channel's weak signal. check them both or see if your neighbors TV acts like yours. If thy neighbor reception is good, then thy TV has poor reception, you can't do anything to fix that. get another TV with good reception qualities.

Your dish is out of alignment. The local channel satellite doesn't have as strong of a signal, so a slight misalignment will cause problems.

It could also be an F-connector needs to be replaced at the dish or multiswitch (if you have one). Sometimes moisture builds up inside the connector and corrodes the copper stinger inside.

If you go into your satellite signal strength menu and find the local channel satellite, you're sure to see either a weak signal or an intermittent one.

One more thing, if a tree has grown in the line of sight between the dish and the satellite, this will cause signal problems too. In this case, you either have to trim the tree or move the dish.

Hope this helps!

P.S.
The customer service reps at DirecTV usually idiots reading from a script with no real field experience.

1. Since my problem (Samsung/Comcast) began with a channel selection button pushed on the TV remote when it should have been pushed on the cable remote, I figured the problem lay with the TV.  Although I was getting the "weak or no signal" msg., the actual signal strength bar graphic showed it to be strong. Disconnecting did no good. I surmised that the TV was simply not recognizing the box.
2. I looked at the manual for my Samsung 750 model. 
3. Page 20 discussed "Selecting the source". I accessed the TV menu and selected "Input", then chose the "HD..." item from the highlighted list. Voila! My tv works again.