1) Is this happened to your new devices or an old one?
2) Is this problem happen to your phone before?
3) Is the notification sound mute only for WeChat or other application as well?
Try checking on the simple step mentioned above and if still cannot (double check to make sure you never mistakenly mute your notification), you might need to try on the steps below:
1) Make sure your 'Power saving mode' is off. If you do use routines like bedtime mode, make sure is off too.
2) Restart your phone or your data/Wi-Fi (power on/ on the airplane mode)
3) Clear cache or history (please make sure you do back up anything you need before you do this).
4) Delete and re-install the application again (please make sure you do back up anything you need before you do this).
FYI, i have the same problem happened before for 'Line Messenger' but after i delete and re-install the application, everything is good.
* Worst case is to format/ reset the whole phone, which i don't really like to do (unless if no choice).
Only is Wechat app, others social app is working fine. Quite hassle as I need click inside then only can see the messages. 😔
I do understand your frustration because i do have this problem on both of my phone previously. Is not only Samsung devices but Apple as well.
Maybe you can try one more step, test out the WeChat notification on different data/ Wi-Fi.
If still no luck and if your device are still under warranty, try book for an appointment with Samsung and show them the problem you are facing as well as telling them the steps you have did before too. On their side, they can check it easily.
* Do not ever go to a normal mobile shop/ repair shop.
Did you also ask for a solution? Because you need to know why it suddenly happen like that and it is very uncomfortable for you to face such issue from time to time.
Ask their advice too because if every time got such problem, what do you need to do? Impossible that you need to visit their center again every time you face such problem. Kinda annoying you know.
I had a similar problem at 1 place and i still don't know why. After I go far from that area, the notification is back. It still happen to me today.