I'm not normally one to recommend complete sets of anything. Usually an artist creates better works and less successful ones. If you have the patience and time to sit through the complete songs of Schubert, for example, you'll find a bunch of duds and lesser versions of masterworks.
The complete choral works of Samuel Barber, an odyssey that Voices of Musica Sacra and their music director, John Kendall Bailey, undertake beginning this weekend, is a bit different. First of all (and maybe most importantly), Barber didn't write hundreds of works and the ones he did publish are impeccably crafted, as Heuwell Tircuit pointed out in an essay for SFCV celebrating the composer's birthday.
But the real reason to seek out these concerts is that you probably haven't heard most of the works that are being given a rare outing here. Prayers of Kierkegaard, The Lovers, Reincarnations, — all of these and many others are beautiful masterpieces that you'll be thankful to have heard live. And who knows when the next chance will come? Voices of Musica Sacra is doing us a tremendous service, and if you go you'll come away amazed and awed by the composer whose reputation should be greater than it is.