I have a Samsung Note 4. Since the upgrade to Marshmallow 6.0.1 the built in Clock app's alarms do not sound consistently in the morning.
This only ever happens in the morning (wake up alarm) making it hard to debug. The phone is plugged in, set to vibrate only, and the Do Not Disturb setting allows Alarms and calls/texts from my favorite contacts.
I get a notification that the alarm was silenced after 1 minute, but there's no way I slept through a minute of it sounding. I cannot reproduce during the day however.
[The alarms in the "Sleep as Android" app work just fine under the same conditions. I have been backing up my Clock alarms with Sleep as Android alarms.]