Like the title describes, I was able to play MM6 for a good 10+ hours without problem. Run smoothly and loading screens were super quick.
Suddenly inside a dungeon during a fight the resolution changed and the game crashed. Wasn't able to start it anymore. I got something along the line of: DDraw set display mode failed.
After trying to find a solution I read about the grayface patch and installed that. After installing it I was at least able to run the game in this little window (changed height and width so that it was bigger). Problem now is that it doesn't run as smoothly as before. There are sound problems, some kind of static and when someone talks it reverberates. During the loading screen the last sound repeats itself until the new map is loaded. Things like that. I mean I can play this way, but I really miss the first 10+ hours with pristine sound :)
I have the suspicion it's the patches fault, but without it I wasn't able to start the game.
Does anybody has an idea how to fix the soundproblem or even how to play the game in it's original state without the patch?
I'm a complete Noob when it comes to Computers and stuff, so I tried to help myself with Google.

Hi Rosaline! Welcome to the world of Enroth and Erathia, I hope you enjoy your stay! :)

Maybe you could try backing up the save games and reinstalling the whole game?

To back up the saves find the .sav files in your game folder, and copy them somewhere else, then uninstall the game, reinstall, and put them back where you had copied them from.

Otherwise, since you said you had a problem with resolution, if reinstall doesn't work, try launching the game's settings launcher and checking if the game resolution is as is should be. However a reinstall should fix whatever happened (delete any leftovers in the folder before reinstalling, make sure you have the .sav files copied elsewhere though!), so run the settings launcher and check what are all the settings and if it happens again, try comparing it to see if the game changed any of the settings. The settings launcher is in the game's folder, by the way.

Thanks for helping :)

The first thing I tried before installing the patch was trying to play around with the compatibility settings. Certain settings let me hear, that somewhere in the background the game started, but I didn't have any window/visuals.

After that I installed the patch and was finally able to get the window to appear and not only the sound. But since the performance was so much worse I in fact did uninstall it (save files secured).
First I started it without putting my old saves back, but I still had the same problem that it won't start. I had to install the patch again just to get the window... and then I put my saves back in the save log. First I thought maybe my save files were corrupted and the reason for my problems, that's why I tried to start it without them, but seems that's not the issue...

I have no idea what to do in that case. Sounds like the game ran into some issues and if a new, completely clean install doesn't work, you're better off contacting official GOG support and asking them. (since so far no one else has provided a solution)

You can also try installing a VirtualBox and running the game there, however first I'd try the GOG support.

Edit: Have you tried googling the issue? Maybe something could come up on the old but still active might and magic fan forums.