On FirefoxOS, sound and vibration will only work if your screen is kept on. C25K Trainer will keep it on automatically. If you turn off your screen, you will not get any run or walk notifications.
Sound
ON
You can turn sound off to stop voice notifications for transitions (e.g. "Start Running"). Please ensure your volume is turned up so you can hear this.
Vibration
ON
Turn on or off vibration for transitions.
Halfway Notification
ON
If you tend to run and turn around half way through, turn on this audio notification which will tell you when to turn around. Sound must be on.