[1] He grew up as the youngest of 14 children of the mining foreman Karl Müller, who from 1882 until his death in July 1937 was the leader of the local miners' music corps (Knappenmusikkorps).
[1] In the 1933/34 season, he was the pianist of the ballet Der Grüne Tisch, worked with Kurt Jooss, also on a tour in Germany and other countries.
[1] In 1948, he was appointed by the Office of Military Government, United States as head of the main music department and at the same time chief conductor of the Stuttgart Radio Symphony Orchestra of Süddeutscher Rundfunk.
[1] He cultivated the broad symphonic repertoire, including contemporary music which had been suppressed under the Nazi regime.
[2] Müller-Kray conducted in 1951 the first complete recording of Schumann's opera Genoveva with the forces of the Staatsoper Stuttgart, including Friederike Sailer in the title role.