1. Concorde can travel at twice the speed of - sound.
2.family needs to have an understanding of what the puppy needs.
3.It serves an excellent "California-Mediterranean" menu.

I want to know why the before speed and no article before sound in sentence 1

and why an before understanding in sentence 2

and why an before excellent menu in sentence 3

I want to know if menu, speed sound and understanding are countable or non countable nouns and what is the rules.