Yes, this is acceptable. It is a form of the future perfect simple construction. Follow the link for more information on that type of phrase.
Because the listening is going to occur in the future, you use "will." By that time, it will be the worst thing you "have heard," with the modifying adverb "ever."
If you leave out "will," it's unclear that the listening will take place in the future:
It might be the worst thing you have ever heard.
That could mean that "you" have already done the listening.
There is one side-note, which is that as constructed, the sentence implies that it might be the worst thing you have heard up to that point. If you want to say that it might be the worst thing the person will ever hear in their entire life, you could say:
It might be the worst thing you will ever hear.