One of the interesting features of Holmboe's later career is that he started writing for some relatively unexpected instruments, as individual performers attracted his attention. Among the instruments that he wrote for are the organ and the accordion.
There are three solo organ works, but so far only two of them have been recorded. Fabula II (Fable II), op.115 (M.268) has been recorded twice. The first recording is by Knud Vad, who premiered the work, and is another recording to be included in the Dansk Musik Antologi series, coupled with several selections from the group of choral works Liber Canticorum (Deutsche Grammophon DMA 028).
The second and more readily available recording of Fabula II is by Eva Feldbæk, and appears on her disc of Danish organ music (Da Capo 8.224018).
Contrasti (Contrasts), op.113 (M.266) has been recorded by Inge Bønnerup, and appears on a collection called Danish Organ Music from the 20th Century, Volume 3 (CD Klassisk 1051). This disc appears to be widely available for download or streaming.
Accordion Sonata No.1, op.143a (M.302) has been recorded twice. The more recent and more widely available recording (particularly for download or streaming, though it also exists on CD) is by Geir Draugsvoll, and appears on Classical Accordion (Simax Classics PSC 1096).
The other recording is by Mogens Ellegaard (who premiered both sonatas), and is coupled on LP with other works featuring accordion by Ib Nørholm, Poul Rovsking Olsen and Per Nørgård (Point P 5058).
Accordion Sonata No.2 'Burlesco', op.179a (M.342) has been recorded once, by Adam Ørvad, and is included on his collection Wing Span (Danacord DACOCD 718).