Classical Purcell, Henry: ''Dido and Aeneas.'' Kirstin Flagstad and Elisabeth Schwarzkopf, sopranos. Thomas Helmsley, baritone. The Mermaid Singers and Orchestra, Geraint Jones, conductor. (Seraphim 60346) - To once hear Flagstad's noble utterances in ''Dido's Lament'' or in any other part of this opera is to never again separate that sound from that music, no matter how beautifully others might sing it. This recording - the outcome of a run of performances of the work in London with the legendary dramatic soprano - became an instant classic, but has been out of the domestic catalog for a while now. It captures all the inherent dignity, womanliness, and heartfelt sincerity that were ever the hallmark of Flagstad's art. The supporting cast is good enough in most cases , the lavish exception being Elisabeth Schwarzkopf's Belinda. This rerelease is a must for anyone interested in the art of grand vocal declamation and noble interpretation.