The waking state, the Common-to-all-men, is the letter a, the first element, from āpti ('obtaining') or from ādimatvā ('being first'). He obtains, verily, indeed, all desires, he becomes first—he who knows this.

In plain words: The sound A stands for the waking state, because A comes first and suggests reaching or obtaining. Whoever understands this gets what they wish for and becomes first.