After completing the shooting of a film you had the roadmap clear for the way forward and the pre release promotions had a key moment in the form of audio release. This function is all about sending a strong message to the public that a movie is arriving.
And it also was the big step in the pre release publicity but slowly the audio functions are vanishing. The reason being, nobody knows how much is the gap between the audio release event and the actual release date for the films because they tend to get postponed for various reasons.
Instead, a pre release function makes more sense because it is held just a week or few days before the release when everything is sure. This has started with the big movies and eventually it could spread onto the other budget movies as well because it is a sensible thing to do.