For a few rare types of hounds (the bloodhound) they were bred with one thing in mind, following a scent trail.
For the vast majority of hounds, they were bred for two purposes, to follow a scent trail, and to bay in different ways so that those following the hounds on horseback or on foot and possibly in the dead of night would know where they were AND what they were doing because they would hopefully bay differently when searching for a trail, a cold trail, a hot trail, animal in sight, animal turning and fighting, or animal went up a tree.
of course there is no gene or set of genes for each of those conditions and what to bay. What exists is a more general breeding toward hounds that were #1 prone to be vocal and #2 prone to be variable and expressive with their noise making.
No one who owns a hound or hound mix should be surprised when their dog makes all sorts of funny noises, any more than someone who has a labrador should be surprised when their dog likes water. There are a few individual hounds that are quiet, and there are a few labs that dislike swimming, but they are the rarity.