This partially reverts commit 1b444a5e23.
Because support for system-level notification sound settings is inconsistent
across Win/Linux/OSX, do the simplest thing that will produce consistent
behavior on all platforms: suppress the system sound by always passing the
`silent: true` option, and play our own sound clip (copied from ios).
// FREEBIE
This removes our custom notification sound in favor of the system sound, and
ensures that the system sound is disabled if the user unchecks the audio
notification option.
// FREEBIE