My thanks to Michael Cervin, who among many others--thank you,
all!--replied to my question:

>>Was "Who is Sylvia?" by Schubert written for solo or chorus?
>
>It is a solo song, D 891 (Was ist Silvia, saget an).  It is possible that
>arrangements by other persons than the composer exist.

By the most amazing fluke, I found a version of this son sung by the King's
Singers in the local library last Saturday!  This is the most interesting
arrangement I have heard thus far.

Unfortunately this song, it appears, is considered one of Schubert's less
important ones.  (The words, I understand, are from Shakespeare, as are the
lyrics of several other Schubert songs.) Fischer Dieskau doesn't sing it on
the one Schubert CD I own --and which I haven't yet unwrapped!

ARch