AIRHORN.mp3 started out as a joke in a group chat. It now has over 11,000 weekly users.
Its sole purpose is to play the famous Airhorn sample when clicked, nothing more.
The Airhorn sample was used in early Hip-Hop in the 1980s. Though according to this Reddit post, it may have even been around since the 1970s, or a similar sample to the same effect.
It is only in the past few years that DJs and producers have given new life to this sample and by doing so have turned it into a popular meme, now adopted by the MLG community. This has attributed to the success of this Chrome Extension.