Do you want to start it as part of your SteamOS desktop session or in 'gaming' mode?

If it's for the desktop session it's simple enough.

Create a new file named "pactl-startup.sh" in a suitable place where it won't get moved / deleted.

Then create the script inside:

Make it executable:
$ chmod +x pactl-startup.sh

Then go to KDE settings > autostart > Add new > Application > Select your pactl-startup.sh file.

It should now run every time KDE logs in.

So I tried the making it executable but I get this error:

(1)(deck@steamdeck ~)$ chmod +x pactl-startup.sh
chmod: cannot access 'pactl-startup.sh': No such file or directory

Oprindeligt skrevet af Hantomei:

So I tried the making it executable but I get this error:

(1)(deck@steamdeck ~)$ chmod +x pactl-startup.sh
chmod: cannot access 'pactl-startup.sh': No such file or directory

No such file or directory means you're not in the same directory as your file or maybe you typed the name of your file incorrectly. If you're using Konsole, you can use the command pwd to confirm you are in the right directory and also command ls to list the files in the directory to confirm the file is there.
instead of using konsole, you can use the file browser(dolphin) right click on your files and in Properties->Permissions you can also change the permissions of the file and make the file executable