code: def switch_to_line_in(self, source=None):
          
          if source:
              uid = source.uid
          else:
              uid = self.uid
  
          self.avTransport.SetAVTransportURI([
              ('InstanceID', 0),
              ('CurrentURI', 'x-rincon-stream:{0}'.format(uid)),
              ('CurrentURIMetaData', '')
          ])
docs: Switch the speaker's input to line-in.
  
          Args:
              source (SoCo): The speaker whose line-in should be played.
                  Default is line-in from the speaker itself.