He collaborated with singers such as Peter Schreier, Aldo Baldin, Ria Bollen, Ursula Buckel, Eva Csapó, Agnes Giebel, Julia Hamari, Ernst Haefliger, Philippe Huttenlocher, Georg Jelden, Helena Jungwirth, Siegfried Lorenz, Adalbert Kraus, Horst Laubenthal, Karl Markus, Barbara Martig-Tüller, Friedreich Melzer, Klaus Mertens, Siegmund Nimsgern, Ernst-Gerold Schramm, Verena Schweizer, Jakob Stämpfli, Ortrun Wenkel, Kurt Widmer and Edith Wiens.
Klein was committed to a variety of projects, and was a member of the jury for the Alois Kottmann Award for classical violin playing along with the singer Agnes Giebel and violinist Alois Kottmann.
Agnes | Ágnes Heller | Agnes Monica | Agnès Varda | Agnes Varda | Agnes Nixon | Agnes Giebel | Agnes Gund | Agnes de Mille | Agnes Chan | Agnes Baltsa | King Edward VII's Hospital Sister Agnes | Hurricane Agnes | Ágnes Szokolszky | Ágnes Szávay | Agnes of France, Duchess of Burgundy | Agnes Mary Frances Duclaux | Agnès Humbert | Agnes Buen Garnås | The Eve of St. Agnes | St Anne and St Agnes | St Agnes | Sara Agnes Rice Pryor | Burton Agnes Hall | Alas Agnes | Agnes Walsh | Agnes von Kurowsky | Agnes Samaria | Agnes of Hohenstaufen | Agnes of God |