This problem became evident to me in December 2016 ... after updating my iPad 2 ( bought April 2013 ) the in- built alarm function of the Apple Clock has been 100% unreliable.
Since then, no Apple updates have fixed the problem. Disgusting.
The problem - the alarm will "sound" but WITHOUT sound.
The only way to deal with this problem is to turn the iPad ( or iPhone, etc. ) OFF, then restart it.
Check - as usual - that the volume is not on " zero" on your Device, and the alarm will work the next morning.
How long the alarm will continue to work after a complete power off then on is with me totally random - perhaps it will work for a month, perhaps only for 2 days. Today my alarm failed after my reset done 5 days ago.
Apple - why don't you fix some of these " simple " faults, instead of mucking around with new stuff. I am fortunate - I have a Samsung 'phone alarm that has never failed; and usually I can ask the hotel to give me a Wake-up call: your alarm CANNOT be relied on !
Let's face it - the Alarm App SHOULD have direct access to the Device volume control, so that it CANNOT be inadvertently turned off either with the volume side button, or with a " Do not Disturb" function.