Hi @Bengt,
first off, your topic title is somewhat misleading. If I understand correctly you already have different sounds for each SIM, but want to be able to change certain system sounds, right? An accurate topic title will help others with a similar problem find it better 
As for your question. I found an app (Ultimate Sound control). It says in its description:
Pro Version Only: replace any system sound with a sound of your choosing, so for example, you can replace your unlock sound with a sound of a door unlocking or a gun cocking.
But I’m not sure if you want to buy a pro version without knowing if this is exactly what you want. (I never tried this app myself).
EDIT: Just read the Pro Version is currently not available on Play Store, sorry I missed that before!
Here I found where your system sounds are stored. After checking on my own FP, you can find the sounds at the following path:
system/media/audio/ui
You need to keep the name of the file, and you need the same format (.ogg). I’d recommend backing up the original sound files somewhere, in case the formatting of the new sound went wrong. I could not find a sound related to Wi-Fi by its name. But maybe one of the sounds you mean is in there.