For the standard OS notification, you are limited to the system sound - you cannot change it and you cannot disable it. However, if the alert is used internally, you might want to take a look at the ToastPrompt control in the Coding4Fun Toolkit - once it is displayed, play the sound.
Also, your playback code should look like this:
SoundEffectInstance currentSI;
using (var stream = TitleContainer.OpenStream("PATH"))
{
var effect = SoundEffect.FromStream(stream);
currentSI = effect.CreateInstance();
FrameworkDispatcher.Update();
currentSI.Play();
}