A quick hack of my database turned up the following (apologies for those
pieces already duplicated):

Alwyn: Sinfonietta fr Strings
Arnold: Sinfoniettas 1,2
Avshalomov
Barlow: Sinfonietta in C
L. Berkeley
Britten (orch and chamber versions)
Dahl
Diamond: Sinfonietta after Sandburg
Herrmann: Sinfonietta fr String Orchestra
Hindemith: Lustige Sinfonietta
           Sinfonietta in E
Janacek
Korngold: Sinfonietta in B
Larsson
Martinu: Sinfonietta giocosa
         Sinfonietta "La Jolla"
Mathias
Miaskovsky: Sinfonietta in a
Moeran
Piston
Poulenc
Roussel
Rubbra
Surinach: Sinfonietta Flamenca
Waxman

I'm surprised to learn of a third Malcolm Arnold Sinfonietta and can't wait
to hear it.

Steve Schwartz