During 1977–78, Schlee was an auditor in Olivier Messiaen's composition class in Paris and studied privately with composer and organist Jean Langlais.
From 1995 on he worked for the Guardini Foundation Berlin and was Scientific Director of the international project “La Cité céleste”, dedicated to Olivier Messiaen.
As a composer, Schlee has written works in all forms, which have been published by Bärenreiter,[2] Lemoine, Leduc, Universal Edition,[3] Doblinger[4] etc.
Jahrhundert, Wiener Musiksommer, Mürztaler Werkstatt, Upper Austrian Stiftskonzerte, Mariahilf-Kultur Graz, Cappella Nova Graz, Copenhagen Philharmonic, Göttingen Symphony Orchestra, Württembergische Philharmonie Reutlingen, International Beethovenfeste Bonn, International Organ Week Nürnberg, Siemens Arts Program, Guardini Foundation Berlin, Theater an der Wien Vienna, University of Music Graz, kunsthaus muerz, Ensemble die reihe, Bachfest Salzburg 2008, Southwest German Philharmonic Konstanz, Staatsorchester Stuttgart, Deutsche Staatsphilharmonie Rheinland Pfalz, Gewandhaus zu Leipzig, International Barocktage Melk, and Musiktage Mondsee.
His music was performed by interpreters like Roger Norrington, Riccardo Chailly, Wladimir Fedossejew, Bertrand de Billy, Placido Domingo, Franz Welser-Möst, Pinchas Steinberg, Dennis Russell Davies, Sándor Végh, Manfred Honeck, Ralf Weikert, Leopold Hager, Christian Simonis, Lothar Zagrosek, Heinz Karl Gruber, Zoltán Kocsis, Philippe Herreweghe, Maurice André, Heinrich Schiff, Ursula Holliger, Ildikó Raimondi, Bernarda Fink, Wolfgang Schulz, Walter Auer, Sharon Bezaly, Michael Martin Kofler, Silvia Careddu, Hansjörg Schellenberger, Marie-Luise Modersohn, Andreas Schablas, Milan Turkovic, Michael Schade, Markus Schäfer, Alexander Krichel, Auryn Quartett, Quatuor Danel, Wiener Streichquartett, Klangforum Wien, Ensemble die reihe, Soloists of Sächsische Staatskapelle Dresden and many others.