On: timer sounds play over your music, so your music keeps going. If your phone's silent switch is on, you won't hear the timer.

Off: timer sounds cut through everything, even the silent switch, but they briefly pause your music.

Locked phone or app in the background? Off is the safer bet: those loud timer sounds tend to keep firing even when your screen is off or you've switched apps. With On, the timer sound may be less reliable once the phone is locked. Either way, a visual flash also fires at zero as a backup.