I basically downloaded a file name custom.mp3 into my isolatedstorage and I can see it via isolatedstorage explorer....
The question here is... How can I access the particular custom.mp3 via URI?
So far I got this.. but I wonder why it is not working:
alarm.Sound = new Uri("isostore:/custom.mp3", UriKind.Absolute);
Your path is wrong. Nothing else is wrong with your code. Post the code you're using for saving the mp3 file in the first place, if you want further help.