Going back to my youth I recall turning up at the Science lab one day to find the Master and his Technician had set up a bell jar on its steel plate with a vacuum pump attached. Inside the jar mounted on rubber bands hung a battery powered electric bell.
The bell was switched on with a loud clattering ringing noise but when the vacuum pump was switched on the noise steadily reduced but looking through the glass the bell was still going full on. Eventually no noise was coming through at all but it wasn’t the glass holding back the sound. When the vacuum was released the sound returned.
Thus it was proved that sound could not traverse any distance through vacuum. This observation made a mess of any theory based on sound such as Doppler Shift so applying Doppler Shift theory to interstellar red shift was totally invalid.