So the problem was that I blindly assumed one can reference to the audio clip with just <wp:Sound>toast.mp3</wp:Sound>
or new Uri("toast.mp3", UriKind.RelativeOrAbsolute)
, as in the MSDN tutorial, when the file is copied to the root of the isolated storage but this isn't the case. You'll have to include the directory where the clip is in your project to the path, in my case Audio\
. Thaks goes to WiredPrairie for pointing this out.
So to wrap things up:
Copy the audio clip which you want to use to the isolated storage with CopyToIsolatedStorage()
from How to play background audio for Windows Phone and follow the instructions in Using custom sounds in toasts on Windows Phone 8 Update 3 . And remember to include the directory structure to the path of the clip or put the clip in the root of your project.