int main (int argc, char **argv) {

    QCoreApplication a(argc, argv);

    for (auto in : QMediaDevices::audioInputs()) qDebug() << "in" << in.description();
    for (auto out : QMediaDevices::audioOutputs()) qDebug() << "out" << out.description();

    // adjust indices for your system
    QAudioDevice indev = QMediaDevices::audioInputs()[1];
    QAudioDevice outdev = QMediaDevices::audioOutputs()[0];

    QAudioFormat format = outdev.preferredFormat();
    QAudioSource *input = new QAudioSource(indev, format);
    QAudioSink *output = new QAudioSink(outdev, format);

    QObject::connect(input, &QAudioSource::stateChanged, [&](auto state){qDebug()<<"input"<<state<<input->error();});
    QObject::connect(output, &QAudioSink::stateChanged, [&](auto state){qDebug()<<"output"<<state<<output->error();});

#if 0
    // this produces silence
    QIODevice *iod = input->start();
    output->setBufferSize(input->bufferSize());
    output->start(iod);
#else
    // this produces buffer repeats
    QIODevice *iod = output->start();
    input->setBufferSize(output->bufferSize());
    input->start(iod);
#endif
    // setBufferSize above has no effect.

}