Function open 

pub fn open(
    device: Option<&str>,
    on_block: impl FnMut(Block<'_>) + Send + 'static,
) -> Result<Capture, Error>

Open the named input (or the system default) and feed on_block its audio, already mono at SAMPLE_RATE, until the capture is stopped.

The device is opened and the stream started before this returns, so a device that will not open is an error here rather than silence later.