Who are the students in Flitwick's choir in Prisoner of Azkaban who are singing the Macbeth song ("Double Double Toil and Trouble")?
The only characters known (or strongly suspected) to be in the Frog Choir are
- Bem (named in the script and closed captions but not actually named in the book or film credits)
- Celestina Warbeck (Named on the old Pottermore site as having been deeply interested in the school's choir. Note that it's not explicitly stated that she was in said choir)
Beyond that, the members are simply listed on the wiki page as "Unnamed players".
Out of universe
The choir members were actually students of the London Oratory School's all-boy "Schola" choir, who also provided the actual singing.
Several (presumably non-singing) female extras appear to have been scattered in amongst them to make the choir look unisex.