Muck's most important recordings were made at the 1927 Bayreuth Festival for the English Columbia Gramophone Company and in 1927-29 in Berlin for the Gramophone Company (HMV). At Bayreuth sometime between late June and mid-August 1927, he conducted about 30 minutes of excerpts from "Parsifal" Acts 1 and 2. His control of phrasing in the Transformation and Grail scenes is regarded as unsurpassed to this day. In December 1927 he led the Berlin State Opera Orchestra in an account of the opera's Prelude, one of the slowest on record. A year later, in December 1928, he made a nearly complete recording of the third act of "Parsifal", using the Parsifal and Gurnemanz singers from that year's Bayreuth performances. The music critic Alan Blyth described this as "the most uplifting, superbly executed reading of Act 3 ... in the history of recording" and Robin Holloway commented that "It realizes better than any other Wagner performance the idea of endless melody". In all about 40% of the opera's score was recorded over the 2-and-a-half-year period. HMV also recorded eight further Wagner orchestral pieces, including the "Siegfried Idyll", with the Berlin State Opera Orchestra in December 1927, May 1928, and November 1929. These have been reissued on various CDs.