File content as of revision 1:0335b5c18618:

// Hello World example for the USBMIDI library

#include "mbed.h"
#include "USBAudio.h"
#include "CircBuffer.h"

Serial pc(USBTX, USBRX);

// frequency: 48 kHz
#define FREQ 48000

// 1 channel: mono
#define NB_CHA 1

// length of an audio packet: each ms, we receive 48 * 16bits ->48 * 2 bytes. as there is one channel, the length will be 48 * 2 * 1
#define AUDIO_LENGTH_AUDIO_PACKET 48 * 2 * 1

// USBAudio
USBAudio audio(FREQ, NB_CHA, 0x7180, 0x7500);

// speaker connected to the AnalogOut output. The audio stream received over USb will be sent to the speaker
AnalogOut speaker(p18);

// ticker to send data to the speaker at the good frequency
Ticker tic;

// buffer where will be store one audio packet (LENGTH_AUDIO_PACKET/2 because we are storing int16 and not uint8)
int16_t buf[AUDIO_LENGTH_AUDIO_PACKET/2];

// show if an audio packet is available
volatile bool available = false;

// index of the value which will be send to the speaker
int index_buf = 0;

// previous value sent to the speaker
uint16_t p_val = 0;

// function executed each 1/FREQ s
void tic_handler() {
    float speaker_value;

    if (available) {
        //convert 2 bytes in float
        speaker_value = (float)(buf[index_buf]);
        
        // speaker_value between 0 and 65535
        speaker_value += 32768.0;

        // adjust according to current volume
        speaker_value *= audio.getVolume();
        
        // as two bytes has been read, we move the index of two bytes
        index_buf++;
        
        // if we have read all the buffer, no more data available
        if (index_buf == AUDIO_LENGTH_AUDIO_PACKET/2) {
            index_buf = 0;
            available = false;
        }
    } else {
        speaker_value = p_val;
    }
    
    p_val = speaker_value;

    // send value to the speaker
    speaker.write_u16((uint16_t)speaker_value);
}

int main() {

    // attach a function executed each 1/FREQ s
    tic.attach_us(tic_handler, 1000000.0/(float)FREQ);

    while (1) {
        // read an audio packet
        audio.read((uint8_t *)buf);
        available = true;
    }
}