Original sentence: They  may also not be able to continue playing or singing with a group when the pitch  drops, or with an orchestra that is not tuned to standard concert pitch A4 = 440 hertz (442 Hz in some countries); this may be due to a perception of pitch which is categorical rather than freely adjustable
Edited sentence: They  may also not be able to continue playing or singing with a group when the pitch  drops, or with an orchestra that is not tuned to standard concert pitch A4 = 440 hertz (variable); this may be due to a perception of pitch which is categorical rather than freely adjustable