void SDL_UnlockAudioDevice(SDL_AudioDeviceID dev)

void MyAudioCallback(void *userdata, Uint8* stream, int len)
{
    printf("The audio callback is running!\n");
    SDL_memset(stream, 0, len);  // just silence.
    printf("The audio callback is done!\n");
}

// don't lock for 2 seconds at a time in real life, please.
extern SDL_AudioDeviceID devid;
SDL_Delay(2000);  // callback runs for 2 seconds.
SDL_LockAudioDevice(devid);
printf("The audio callback can't be running right now!\n");
SDL_Delay(2000);  // callback doesn't run for 2 seconds.
printf("Ok, unlocking!\n");
SDL_UnlockAudioDevice(devid);
SDL_Delay(2000);  // callback runs for 2 seconds.