Kyle Boreing makes an interesting observation—four of the five song clips he’s listened to on the Oak Ridge Boys’ latest CD are in the key of E. David Bruce Murray adds that on four consecutive songs on Brian Free and Assurance’s latest CD are in the key of D. [EDIT, 6/6/12: Broken link removed.]
They pose the question of which other albums do this, but I think a far more interesting question is why. I think the answer is fairly simple. Some of this happens at random, but many song key decisions are based on the singer’s ranges. Chances are that Jeremy Lile of Brian Free and Assurance has a lot of confidence and a pleasing vocal tone at the low D, Bill Shivers is comfortable at the D above middle C, and Brian Free is comfortable at the F-sharp above that. Ditto with the Oak Ridge Boys a key higher, except that I think it may be that their bass is comfortable at low E and their tenor is often singing E above middle C.
Professional singers do pay attention to these sorts of things. One tenor singer, for example, has told me that he keys his signature song in the fairly obscure key of F-sharp because the third above F-sharp (A-sharp/B-flat) is the power note in the song, and B-flat is his personal strongest power note.
Now some groups may have a tenor who is confident at high A-flat and a bass who’s happiest at D. They would probably have more variety in their arrangements. But when groups are fortunate enough that their singers all click in the same key, don’t be surprised to see them using that key (and its fourth and fifth, e.g., A, D, and E) frequently.