But since it's also somehow a question I hope it's ok in here.)
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Edit October 15th, 2013: With the GDR3-Update you get this feature built-in, so no more need to do this then!
while AlvinPhilemon found out a registry tweak to set custom sounds on the Samsung Ativ S (as explained in this post), it is not quite practical, since you can't see the sound in the list with the others, so you can't choose or switch them without always edit the registry.
I found another way to do this, and I think it's a bit more elegant (see attachment to have a look), although it only can change the sms, e-mail or mailbox sounds.
This is what you need:
1. Interop Unlock
2. Registry Editor (use SamWP8 Tools)
3. Full File System Access (use SamWP8 Tools)
4. Your Notification sound (I tried mp3-, wav- and wma-format, all worked)
This is how you do it:
1. On your PC, browse to your Phone\PROGRAMS\CommonFiles\Sounds\ and paste your new sound there.
2. To make it visible in your sounds selection list inside the Settings-app you have to create a registry entry: Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shell\OEM\So unds\Notifications\ (This is the Samsung specific location for notifications; I tried the whole thing with the standard Microsoft sound location HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shell\Sounds \Notifications\ but it didn't work). In this registry location you can see a list of the the existing Samsung notification sounds ("Knock.wma", "On time.wma" and "Whistle.wma") and how their name is displayed in the Settings-app ("Knock", "On time" and "Whistle").
3. Create a new key and set the Value to the name of your soundfile (for example "Silvestro.mp3"), the Type to String and the Data to the name how you want it to be displayed. There might be an error notification after writing, but just read the key to check you wrote it correctly.
4. Done! Go to the Settings-app and choose the newly created sound as message, mailbox, or e-mail-sound!
What do you think of this solution? Of course it would be nice to pack this feature in an app that first lets you choose a soundfile from the Public or SD-Folder, then copies it, then creates a registry entry and then also is able to delete those things an put everything back to normal, but since I'm just starting with C# there's no way I can do this myself. Anybody interested in trying?
Thanks for your comments!