I have found out why the alarm clock did not work for me. At the time it did not work I almost tried to wipe out the OS but the reason was very simple: one of the audio app has audio-disabled then is closed; it would interfere with the audio and causes the alarm not working.

So try to open whatever audio app you have to see if it has disabled sound or not. If it did, restore sound and close it. Alarm clock should work; at least, that worked for me.

Good luck !