teensy-audio-tuner/examples/Simple_Sine/Handle_Commands.ino

void handleCmds( String cmd ) {
  String p = cmd;

  if (p.startsWith("f ")) {
    p.trim();
    p = p.substring(2);
    float t = p.toFloat();
    Serial.print("new frequency: ");
    Serial.println(t);
    //AudioNoInterrupts();  // disable audio library momentarily
    sine.frequency(p.toFloat());
    //AudioInterrupts();    // enable, both tones will start together
  }
  else if (p.startsWith("a ")) {
    p.trim();
    p = p.substring(2);
    float t = p.toFloat();
    Serial.print("new amplitude: ");
    Serial.println(t);
    AudioNoInterrupts();  // disable audio library momentarily
    sine.amplitude(p.toFloat());
    AudioInterrupts();    // enable, both tones will start together
  }
}