Mute them and put them in "Do not disturb" so they don't vibrate either? What other noise does your phone make?
The only sound that plays is sound that you specifically asked for, like music or sound from a video, the alarm and sound from badly behaving apps.
"Unfortunately, where you listed the example of 'badly behaving apps' as one of the sounds it does play, this shows that in many cases the only way to be sure that it will not play sounds is to turn the power off. That is the only way to know this will not happen."
Maybe it's a quirk of this particular model (iPhone 5), or some sort of bug, but it's definitely making the low battery sound while physical switch is set to mute.