Why do I have picture but no sound on my hisense v77 series ltdnv77us model

See page 32 of user manual (Quick Problem Solving Tips). Your problem is discussed there.

50A6G-User-Manual.pdf (hisense-usa.com)

1. Why hisense TV red flashing power light has sound but no picture
There could be a few reasons why your Hisense TV has a red flashing power light but no picture. It could be an issue with the TV itself, the power supply, or the input signal. Let's take a look at each of these possibilities in turn.

2. How to fix hisense TV red flashing power light has sound but no picture

If your Hisense TV has a red flashing power light but no picture, there are a few things you can try to fix the problem. First, check to make sure that the power supply is plugged in and that the TV is receiving power.

Next, check the input signal to make sure that it is compatible with the TV. Finally, if the TV itself appears to be the problem, you can try resetting it.

3. What to do if hisense TV red flashing power light has sound but no picture

If your Hisense TV has a red flashing power light but no picture, there are a few things you can try to fix the problem. First, check to make sure that the power supply is plugged in and that the TV is receiving power.

Next, check the input signal to make sure that it is compatible with the TV. Finally, if the TV itself appears to be the problem, you can try resetting it.

No it will not be the tcon board clicking and if you see the logo that board is working. Your going to need to get some voltage readings from the power board and see if it's working ok for starters. Next would be unplug the backlight connector and see if the sound goes away, if it does you have bad backlights and I think this will be the issue

Most likely neither, on the majority of Hisense sets it has been bad LED backlights

I'd like to help but can't find that model number. Try other available numbers after clicking the magnifying glass icon (search pane) on this page:

https://www.hisense-usa.com/support

Check the backlight circuitry

Throw it away. Seriously most of the parts will wind up costing more than a replacement set...and you are taking a chance that they will even fix it. Hisense has a poor reputation for customer service and does not make service info readily available. If you do decide to try and repair it....yu probably need at minimum an led driver board...assuming the led is not bad.

Fault with main board. Damaged component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/ . In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

Unplugging the tv resets it and the remote works fine afterward. It happens now and then when you push too many buttons at the same time on the remote