Do not send extra channels for aggregate devices.

src/backend/mod.rs

@@ -3612,16 +3612,11 @@ impl<'ctx> CoreStreamData<'ctx> {
                 self.stm_ptr,
                 input_hw_desc
             );
-            // Notice: when we are using aggregate device, the input_hw_desc.mChannelsPerFrame is
-            // the total of all the input channel count of the devices added in the aggregate device.
-            // Due to our aggregate device settings, the data captured by the output device's input
-            // channels will be put in the beginning of the raw data given by the input callback.
-
-            // Always request all the input channels of the device, and only pass the correct
-            // channels to the audio callback.
+            // The input stream channel count is not necessarily the same as the
+            // input **device** channel count; request the latter from audiounit.
             let params = unsafe {
                 let mut p = *self.input_stream_params.as_ptr();
-                p.channels = input_hw_desc.mChannelsPerFrame;
+                p.channels = device_channel_count;
                 // Input AudioUnit must be configured with device's sample rate.
                 // we will resample inside input callback.
                 p.rate = input_hw_desc.mSampleRate as _;