The sound is a low rumble / drone usually from about 40mph upwards. Mine was a front left , it would go quiet when turning left, as the bearing became loaded.
In terms of changing all 4, NO!, why change them if they aint broke . If your mechanically minded, and have access to jacks etc, and a good puller, you could do them yourself. to determine if its a front L or R bearing which has failed, find a roundabout which will let you make left and right hand turns, drive round the roundabout at speed, and see which way makes the drone / rumble go away , if turning L makes it silent, its your left bearing and vice versa for the right.
a good indy garage should be able to do one for a reasonable price, if i can find my disc, i will give you the book time
Mart