Compiler doesn't like function code
The clue is in the error...


  too few arguments to function 'Adafruit_VS1053_FilePlayer onSignalSound(Adafruit_VS1053_FilePlayer, const char*) 


You have defined the function as this:

Adafruit_VS1053_FilePlayer onSignalSound(Adafruit_VS1053_FilePlayer mp, const char* fileName) {


And yet you then try and call it like this:

onSignalSound();


Spot the mismatch?

I suspect you don't want to be calling that function there since it seems to be in a bit where the sound has already been started.