Paucity of sources and my inability to read Flemish again render this list incomplete.
AUGUST DE BOECK: A CATALOGUE OF THE ORCHESTRAL MUSIC
1893: "Rapsodie dahomeenne" for orchestra: 5 minutes + (Marco Polo, Talent and Etcetera cds)
1895: Ballet "Cendrillon"
Symphonic Poem "Assepoester"
1896: Symphony in G: 31 minutes + (Marco Polo, Discover and Cypres cds)
Ballet "La Phalene"
1910: Cantata "Gloria flori" for children's choir and orchestra
1913-20:Suite from the Opera "Francesca": 46 minutes + (Phaedra cd)
1923: Fantasy on Two Flemish Folk Songs for orchestra: 6 minutes + (Etcetera and Naxos cds)
Cantata "De zangers in het woud" for two child singers and orchestra
1926: Fantasy for Oboe and Orchestra + (Phaedra cd)
1926-29:Piano Concerto: 23 minutes + (Phaedra cd)
1929: Violin Concerto: 26 minutes + (Marco Polo and Etcetera cds)
1931: Nocturne for orchestra: 8 minutes + (Etcetera cd)
1935: "Bondslied" for choir and band
1936: Cantilene for Cello and Small Orchestra
1937: "In der Schuur" for orchestra: 5 minutes + (Etcetera cd)
and
Inaugural March for orchestra
Elegie for string orchestra + (Phaedra cd)
Gavotte for string orchestra
Cantata "Huldezang aan Maria-ter-Noord" for chorus and orchestra
"De mooiste maand is mei" for two vocalists and orchestra
"Het lied van de leeuwerik" for two vocalists and orchestra
"In het woud" for mezzo-soprano and orchestra
"L'eglise paysanne" for middle voice and string orchestra
and other songs with orchestral accompaniment and works for brass band
Source:
http://www.augustdeboeck.be/downloads/Muziekacademie_ADB_ASSE.pdf (http://www.augustdeboeck.be/downloads/Muziekacademie_ADB_ASSE.pdf)
The 1923 Fantasy on two Flemish Folksongs exists also on Naxos 8.550584 (A.Rahbari cond., BRT Phil.Orch.).