Recently, I wanted to know how I could turn of the startup sound on the Steam Deck. After a bit of searching I found that there is not an option for that, but since it's just a video, it can be replaced with a silent one.
There are 3 ways to replace the boot animation:

• Using a movie from the Steam Points Shop.
• Using the Animation Changer plugin for Decks Loader.
• Use an own movie.

In this walkthrough I want to explain how you can set your own movie as the startup movie.

Here are the steps:

1. Open the Steam menu by pressing the Steam button or the B button.
2. Select the Power option.
3. Select Switch to Desktop.
4. Open the file explorer. I'm assuming it's Dolphin.
5. Select Home in the top left corner to go to your home folder.
6. Open the hamburger menu in the top right and make sure the Show Hidden Files option is checked.
7. Open the .steam folder. Next, open the root folder and then the config folder.
8. If you see an uioverrides folder, open it. Otherwise, create it first.
9. Inside the uioverrides folder should be a movies folder. If it's missing, create and then open it. You should now be in /home/deck/.steam/root/config/uioverrides/movies/.
10. Place the movie in this folder. It should be in the webm file format and be less than 30 second long.
11. Open the hamburger menu in the top right, uncheck Show Hidden Files, and close the file explorer.
12. Double-click on Return to Gaming Mode or restart the Steam Deck.
13. Open the Steam menu.
14. Select Settings.
15. Select Customization.
16. Select your movie.

Now you should have your own movie show up when your Steam Deck starts.

Two more things.

• You can skip the startup movie by pressing any button on the front or top of the Steam Deck.
• You can create a movie out of a single image using ffmpeg -loop 1 -t 5 -i startup.jpg startup.webm. Here, -loop 1 means the same image will be used for every frame, -t 5 means the movie will be 5 seconds long, -i startup.jpg is the input image, and startup.webm is the output movie.