PluggableUSBAudio not working

Trying this example:

#include "PluggableUSBAudio.h"
USBAudio audio(true, 44100, 2, 44100, 2);
static uint8_t buf[128];

void setup() {
  for (int i = 0; i<sizeof(buf); i++) {
    buf[i] = 128 * sin(i);
  }
}
void loop() {
  audio.write(buf, sizeof(buf));
}

Compiles without error but the Pico continues to enumerate as a USB Serial device, not as Mbed Audio.
Thanks, 
Bob