This is odd because this symbol is defined in Adafruit_VS1053.h so this redefinition should generate a compiler error. The interrupt pin number is defined by DREQ in the original library.
#define VS1053_FILEPLAYER_PIN_INT 18 // Added 10-31-2017 QK.
If you are using a modified fork of the Adafruit library, revert to the original version. I suggest posting your sketch code to get more help.
Try adding Serial.print messages in the library to verify the code is doing what you expect. The useInterrupt function has commented out a Serial.print message which will tell you whether the interrupt function has been attached. Uncomment it and see if the message appears or not. If not, it has something to do with VS1053_FILEPLAYER_PIN_INT.
The function feeder is the interrupt service routine so to determine whether it is being called or not, add some digitalWrites to toggle a GPIO pin. Hook up an LED and see if it blinks or not. If it does not, the interrupt attach failed.