It's definitely not your spec, it's the game, but make sure you use the latest Nvidia drivers, and try betas too. Also make sure your audio driver is updated too.
First off, the game is a RAM hog, so disable any background programs you can and reboot before playing to free up RAM. Also disable desktop composition and themes via the game's shortcut (right click shortcut, properties, compatibility).
Try running in Windowed Borderless mode vs Fullscreen, and try it with and without VSync. For some this has greatly decreased or eliminated cutscene stutter. It may only stop or minimize the audio part of it though, as the out of sync video on load scenes seems to be specifically a memory leak problem with the game.
If you continue to get out of sync video on load scenes, save and quit after the mission starts, then relaunch the game. The first load scene always plays in sync due to refreshed RAM. Restarting the game can go a LOT quicker if you disable the intro cutscene where the commander is first talking to all his troops at the start of the game.
You can disable the intro cutscene by making a blank text file and renaming it. First copy and relocate the original seoul_load.bik file in the main game directory. Give the text file the same name with the bik vs txt extension, then place it in the main directory.
Alternatively you can download Bink Player, minimize the game, search for the game's bik file for the mission you just started, and play that file straight through Bink Player to see it without the sync problems. This may or may not exacerbate the memory leak problem in game, but most with 16GB RAM and that powerful a GPU don't experience problems in game.
Sorry, but these seem to be the best and only workarounds until the game gets a much needed optimization patch. As far as I know though Nvidia may be ahead of AMD on drivers for this game, assuming you've tried the latest one.
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