For starters, unplug the TV and the Dish network box's power cords for a full 5 minutes to reset both micros. Plug them back in, wait for the box to reboot and check your sound.
If it's still floating up and down, depending on your TV model, you may have the Audio Leveler on your TV set turned on, and it needs to be turned off.
That's a common issue causing the volume to go up and down while watching TV.
I can verify if you have that feature, if you can get the MODEL number off the sticker on the back of the TV. While you are there, also get the Serial number. That's needed if parts need to be replaced.
Ok, that model had several main boards used during manufacture, and so the part can only be replaced if we know which main board was used during manufacture...
Remove the rear cover by laying the TV face down on a blanketed table. Remove all the screws around the outer edge of the TV set, remove the stand, and remove the screws located between the TV's input jacks. You should be able to remove the cover.
Then look at the board that has all the input jacks on it. There are some white paper tags on it. The number you see on that tag will partially match this list of numbers for the main board, get that number and I can look up the part number you need, ok?
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Here's the list of numbers to compare it with --
Get back with me and give me the number you find that matches.