What is the fastest way to reach enlightenment? I am asking for reference request and sutras where a person can cut through samsara quickly and obtain liberation.

I have heard a story when Buddha was asked this question and he replied "Sound". However I haven't found any reference of this &amp; don't know how much accurate this is.

Thoughts are more than welcome!
You're probably thinking of the Bahiya Sutta (Udana 1.10).

Once there was a bark-clad ascetic Bahiya who thought he was enlightened, but a deva told him that he wasn't and asked him to seek out the Buddha for his guidance.

He found the Buddha who gave him a brief instruction, which resulted in his near-immediate enlightenment. Very shortly after that, he was killed in a freak accident. The Buddha said that Bahiya became totally unbound (i.e. enlightened).

What did the Buddha teach him? (translated by Thanissaro)


  "Then, Bāhiya, you should train yourself thus: In reference to the
  seen, there will be only the seen. In reference to the heard, only the
  heard. In reference to the sensed, only the sensed. In reference to
  the cognized, only the cognized. That is how you should train
  yourself. When for you there will be only the seen in reference to the
  seen, only the heard in reference to the heard, only the sensed in
  reference to the sensed, only the cognized in reference to the
  cognized, then, Bāhiya, there is no you in connection with that. When
  there is no you in connection with that, there is no you there. When
  there is no you there, you are neither here nor yonder nor between the
  two. This, just this, is the end of stress."


Alternative translation from here (Ireland):


  "Herein, Bahiya, you should train yourself thus: 'In the seen will be
  merely what is seen; in the heard will be merely what is heard; in the
  sensed will be merely what is sensed; in the cognized will be merely
  what is cognized.' In this way you should train yourself, Bahiya.
  
  "When, Bahiya, for you in the seen is merely what is seen... in the
  cognized is merely what is cognized, then, Bahiya, you will not be
  'with that.' When, Bahiya, you are not 'with that,' then, Bahiya, you
  will not be 'in that.' When, Bahiya, you are not 'in that,' then,
  Bahiya, you will be neither here nor beyond nor in between the two.
  Just this is the end of suffering."


This is a teaching on anatta.

However, this may not work for everyone as it did on Bahiya, because Bahiya apparently was already advanced in attainment. Please see this answer.

For complete beginners, study of the Dhamma (in order to cultivate Right View) and practice of the five precepts and virtue (sila) is recommended. Please see this answer.