As you could see in the first post, I had this problem before.
After reflashing my N950 it was ok again. but then, some weeks later it started to behave odd again
I do not have a bluethoot device, so that cant be the reason for me.
What I figured out:
When the device is upside down, the alarm sound works correctly. How ever it goes directly into the OK/Snooze dialogue without showing the bumping lock screen. When it is upside (up), it also goes directly into the OK/snooze dialogue. The music plays, but stops after a second. This even behaves like this when the device is covered.

On my N9 I have almost the same software installed, but there it works fine all the time.

Something else I figured out:
When I get new messages/SMS,... they are listed on the lock screen. How ever, it only says "new message" instead the showing the text of the message.