Compiler doesn't like function code

I am getting this error from the Visual Studio compiler (I use the Visual Studio add-in Visual Micro for Arduino).

ModelRailroadCrossingSignalController.ino:In function 'void loop() ModelRailroadCrossingSignalController.ino:210:15: error: too few arguments to function 'Adafruit_VS1053_FilePlayer onSignalSound(Adafruit_VS1053_FilePlayer, const char*) :onSignalSound() ModelRailroadCrossingSignalController.ino:note declared here :Adafruit_VS1053_FilePlayer onSignalSound(Adafruit_VS1053_FilePlayer mp, const char* fileName) { Error compiling project sources

Here is the function and loop (all the defines and instances left out for brevity, they aren't the issue

void loop() {
// put your main code here, to run repeatedly

signalEntrySensor = checkEntrySensor();
if (signalEntrySensor == true) {
    if (crossingSignalInOperation == false)
    {
        Adafruit_VS1053_FilePlayer mp = onSignalSound(musicPlayer,"Trainxng.mp3");  //all other functions are called in the sound routine?
        if (debugMode)
        {
            Serial.println("Sound byte called");
        }
        while (mp.playingMusic) {
            // file is now playing in the 'background' so now's a good time
            // to do something else 
            offRoadPower();
            lowerSignalArms();
            onSignalSound();
            startSignalFlashers();
            crossingSignalInOperation = true;
        }                   
    }
}

Adafruit_VS1053_FilePlayer onSignalSound(Adafruit_VS1053_FilePlayer mp, const char* fileName) {
// Play the train crossing sound file
if (debugMode)
{
    Serial.println(F("Playing track - "));
    Serial.print(fileName);
}
//all sound files are on the micro sd card on the music player shield
if (!mp.startPlayingFile(fileName)) {
    Serial.println(F("Could not open file "));
    Serial.print(fileName);
    while (1);
}
return mp;  }

I'm a C# guy, not ever using C++ until now, but this code should certainly work, not sure why the function is not compiling.