After update 1.3.1 I noticed the constant sound of 'Thunder' in the background, as if it was getting ready to storm, or was storming. But this was happening All-The-Time, no matter where I was and even during cut scenes, and dialogue. Very annoying.

But after playing 'The Siege of Paris' (Trial by Fire), the constant 'thundering' in the background stopped. To be percise, it stopped after 'Trial by Fire'. Very odd. But, I did notice the 'Trail by Fire' had a cut scene where the queen was to be burned at the stake, and a very pronounced lightening/thunder storm occurred while the scene unfolded, even panning to the sky showing flashes of lightening.
Now, after that scene, the constant 'thunder' stopped. 
I went on and tested other realms, also proving it had stopped across all over locations.

Just an observation I wanted to share, so if you are suffering from this bug, and have not reached that cut-scene yet, hopefully it will stop for you as well.

Update version: 1.5.2

If you have constant thunder

I have just finished the DLC Siege of Paris and here is what I found:

As soon as you talk to Count Odo as first action in the quest "Fire and Faith" or at the point where you search for somebody in a church there will be a chance for constant thunder to be added to your game sounds. I tried it with several saves from that quest line and that is when the thunder starts. At first it is only in the game play. And wherever you go it will be there. Layla has it, it will be in any realm you visit (don't know about Dawn of Ragnarok realms since I don't own that DLC). Then it will also be in the menus (inventory, map, etc). You won't be able to escape from it.

But don't worry, your sound files themselves won't get damaged. The sound will just be overlayed. As soon as you exit the game and exit UbiConnect and then restart and load a save from before the thunder will be gone. But the moment you load a save where you had already started Fire and Faith you will hear the thunder right in the loading screen of the save game.

I tried everything: start the game with a non-thunder save, then start the save I was working on. Do it the other way around. Reboot my system. Verify all game files. Even wait for a thunderstorm in Raventhorpe and then ride away from it. The thunderstorm itself stopped but not the thunder noise lol.

The only way to get rid of the thunder noise being overlayed is: finish the quest Fire and Faith, then play through the next quest Trial by Faith, which leads you to a big confrontation. You have to play through that. Once you are finished with the confrontation, the constant thunder will stop.

I have tried to continue the game from a save within Fire and Faith and went to a new shire in England but was so annoyed at the constant thunder for hours after hours that I stopped and went back to Francia and finished that part of the DLC.

Once you are done with the confrontation (whatever you decided to do, I tried both options), you no longer hear the constant thunder.

I have now played a whole shire arc in Eivor's story and the thunder has really stopped for good.