Under her tutelage, he continued his vocal training from 2008 at the Hochschule für Musik und Theater München, and at the opera class at Bayerische Theaterakademie August Everding, based at the Prinzregententheater.

Appl has given recitals regularly at Wigmore Hall in London and the Schubertiade, and has performed at major venues including Festspielhaus Baden-Baden, Concertgebouw in Amsterdam, Elbphilharmonie in Hamburg, and the Musée du Louvre in Paris.

He has collaborated with pianists Graham Johnson and James Baillieu, and has also performed recitals with Kit Armstrong, Kristian Bezuidenhout, Helmut Deutsch, Julius Drake, Boris Giltburg, Pavel Kolesnikov, Simon Lepper, Malcolm Martineau, Wolfram Rieger, Martin Stadtfeld and Roger Vignoles.

In an interview in 2024 Appl commented that the art song is "just the highest achievement I can have as an artist, in terms of being my own stage director, my own conductor, being in charge of every decision in every moment.

The feeling of nakedness, of the direct communication, facing the audience, seeing them, not having the bright stage lights like in opera, or the pit between — it’s a brutal connection you have to have with the audience..."[4] While studying at the August Everding Academy, Appl took part in opera and operetta productions, including appearing as Ypsheim-Gindelbach in Wiener Blut,[5] Falke in Die Fledermaus by Johann Strauß, Schaunard in Puccini's La bohème and Baron Tusenbach in Tri sestry by Peter Eötvös.

[9] In 2021 he was Harlequin in Ariadne auf Naxos at the Gran Teatre del Liceu, as well as Jesus in a semi-staged St John Passion at the Théâtre du Châtelet in Paris.

[12] His debut album as an exclusive Sony Classical recording artist, "Heimat", won the Prix Dietrich Fischer-Dieskau (Best Lieder Singer) at the 2017/18 Académie du disque lyrique [fr] Orphées d'Or.

He has been fortunate enough to have performed works written specifically for him by composers including Kit Armstrong,[16] Marian Ingoldsby,[17] György Kurtág,[18] Nico Muhly, Susan Oswell[19] and Matthias Pintscher.

[32] In spring 2020 Appl was involved in a new movie project called Breaking Music, which was filmed during his visits of Buenos Aires and Berlin to discover tango, its history and similarities with as well as differences to Lieder.