I believe it has something to do with surround
sound.

THX was short for Thomlinson Holson's Experiment and it also refers to Luca's THX_1138.

George Lucas was disappoinited that his films didn't look or sound as well as it should in movie theatres.

So THX is kind of a good housekeeping seal of approval.

THX certifies movie theatres so that it meets certain requirements of performance.

Sound proofing, quality of speakers power etc.

Later, THX is used to certify everything from surround sound receivers, to speakers and DVD/LaserDisc.

Can something be good that isn't certified? of course....but THX certification means that it should meet a certain level of quality.