From 1937 to 1940, Stephani studied at the Berlin University of the Arts with Walther Gmeindl, Fritz Stein, and Kurt Thomas and became a "Kameradschaftsführer" in the National Socialist Student Union there.
Stephani was involved in establishing the Waffen-SS's own musical organization, including a symphony orchestra, and probably played a significant role in the 1942 decree concerning "works by 1.
Jewish, 2. undesirable, and 3. suitable composers for the Waffen-SS.” When he married the singer Hanne-Lies Küpper on December 28, 1944, a ceremony conceived by Stephani himself according to SS rites took place.
In 1968, a proposal by the Minister of Culture of North Rhine-Westphalia to award Stephani the Order of Merit of the Federal Republic of Germany was rejected due to his former position in the SS.
From 2015 to 2018, on the initiative of the Detmold University of Music, Stephani's activities during National Socialism and in the post-war period were historically reviewed.