message: AudioMixer: enable mix even if no chan 0
diff: void AudioMixer4_F32::update(void) {
  audio_block_f32_t *in, *out=NULL;
  - out = receiveWritable_f32(0);
  - if (!out) return;
  -
  - arm_scale_f32(out->data, multiplier[0], out->data, out->length);
  -
  - for (int channel=1; channel < 4; channel++) {
  - in = receiveReadOnly_f32(channel);
  - if (!in) {
  - continue;
  + //get the first available channel
  + int channel = 0;
  + while (channel < 4) {
  + out = receiveWritable_f32(channel);
  + if (out) break;
  + channel++;
  }
  + if (!out) return; //there was no data available. so exit.
  + arm_scale_f32(out->data, multiplier[channel], out->data, out->length); //there was data, so scale it per the mier
  + //add in the remaining channels, as available
  + channel++;
  + while (channel < 4) {
  + in = receiveReadOnly_f32(channel);
  + if (in) {
  audio_block_f32_t *tmp = allocate_f32();
  arm_scale_f32(in->data, multiplier[channel], tmp->data, tmp->length);
  @@ -21,6 +25,10 @@ void AudioMixer4_F32::update(void) {
  AudioStream_F32::release(tmp);
  AudioStream_F32::release(in);
  + } else {
  + //do nothing, this vector is empty
  + }
  + channel++;
  }
  if (out) {
  