The Kennedy Center and Wolf Trap have ignored the 50th anniversary of "Show Boat," but Burn Brae Dinner Theater is celebrating Jerome Kern's rich score (and introducing two fine, fresh voices) in a revival that opened Wednesday night.

To Magnolia, daughter of the captain of the "Cotton Blossom," and Gaylord Ravenal, the riverboat gambler. Oscar Hammerstein II and Kern assigned such duets as "Make Believe," "You Are Love" and "Why Do I Love You?" - as romantic a trio of songs as any of our musicals can boast.

Beth McVey., a graduate of the Cincinnati Conservatory of Music and, as Miss West Virginia, a contestant for the 1973 Miss America, makes her area bow as Magnolia. It is as lovely a performance of the songs as ever you're likely to hear. Hers is the golden soprano type, finely trained for "After the Ball" and effortless in the upper register.

Thomas Machen, an Arizona instrumentalist in one of Michigan's Interlochen Music Camp summers, was encouraged to drop his trombone in favor of his baritone. Vocal studies in Vienna and the Metropolitan Opera Studio followed. He brings an exceptionally well-trained voice to Ravenal.

Such fine young singers reflect the hidden riches of America's talents. The dinner theaters serve a vital need, if only to allow them the experience to develop. Burn Brae, an out-growth of the failure of Washington's American Light Opera Company, has flourished for 10 years as a showcase for telents under the leadership of John Kinnamon and Bernard T. Levin.

Burn Brae's orchestral accompaniment may seem off-putting but it works. It is a taped orchestra instead of the usual small, live band, and is an economically viable compromise.

Hammerstein admitted trouble in compressing Edna Ferber's sprawling novel, as Miles kreuger details in his absorbing anniversary volume on "Show Boat." Burn Brae follows his excision of Magnolia's daughter, Kim, for the 1946 revival.

Another lovely voice is Constance Dameron's, alternating with Dorothy Krikorin, as Julie for "Can't Help Lovin' Dat Man" and "Bill," originally in a low, Helen Morgan register. Dameron's silvery soprano, well-projected, graces the songs effectively. George Bussrd, the D.C. policeman whose "Ole Man River" was revealed in an ALOC production, remains a compelling bass 16 years later, alternating with Charles Lee.

Phyllis Goldblatt's assured Parthy, Dick David's Andy and the Frank of Rodney Fayman, who also directed, are solid performances in the company of 25.