A brief search of the catalogue of the library at the Royal Academy of
Music in London, reveals the following works by Volkmann:

Serenade No3 in d minor for string orch & solo cello
Serenade No2 for string orch, arr for cello & piano (Schotts 1932)
Chant du Troubadour Op 10 Violin or cello & piano (Augener 1900)
Schlummerlied for viola, cello & piano Op 76 (Schott 1882)
Piano Trio in B minor Op 5 (pub Budapest 1854)

also several editions of the A minor cello concerto, some string quartets
and orchestral works.  I am a viola professor at the Academy and could
enquire about copying or loaning any of the above if it would be of help.

Best wishes,
Paul Silverthorne