Is there any meditation technique to be aware of what you hear? I have observed that while meditating if I turn my awareness from breath to 'what I am hearing' I have a heightened sense of present moment awareness. If I listen mindfully to what I am hearing the mind invariably comes in present moment. As the hearing can only happen in present moment. Also, I see that there are foreground noises and constant background hum, like a low-frequency sound in the background. 

I want to ask is there any meditation technique in Vajrayana or any other Buddhist tradition to remain aware of the hearing and transcend the sense spheres thereby? If yes, what is it and how to practice it?
Yeah, you could probably make that happen.  What you’re looking for is a sense sensation that has some stability.  Visually, you would use something like a kasina, but I see no reason why a sound kasina couldn’t also work provided that you’re diligent about it.  As to what this sound kasina would be made up of, you’d want something that was consistently present.  A dripping faucet would probably work, but I’d be more inclined to use something like that background hum you were talking about.  You could also plug your ears and listen to the rushing sound of your blood in your ears.

Remember though, the whole point of a kasina is to generate a counterpoint sign.  When that arises, you would abandon whatever sound you were working with.