The score for Believe was originally conceived to be performed by four musicians: Piano, violin, cello and harp (with the harpist doubling on the accordion part, which could be played on a keyboard). This number of musicians could be increased or decreased depending on production and budgetary needs, but at least four is highly desirable.
Minimally, since two of the major characters play musical instruments as part of the story (Father plays accordion and Orpheus plays violin), there would need to be at minimum two musicians: a keyboardist (who would double on Piano and Synthesized Accordion) and a violinist (unless the actor playing Orpheus also happens to be an accomplished violinist and can play his part on his own).
There is a piano/vocal score available for rehearsal or for performance of individual songs.
The quartet arrangements (Piano, Violin, Cello, Harp/Accordion) are currently being written by the composer. The following MP3 recordings are midi renderings to serve as examples as to what the musical accompaniment would sound like as intended.
The following samples are musical accompaniment only (no vocals):