As I understand it, the twin paradox was integral to Einstein's conclusion that nothing can travel faster than the speed of light.
Why wasn't there a similar conclusion for a hypothetical scenario where 2 blind musicians play the trumpet whilst moving away from each other at a speed close to the speed of sound. They would obviously hear a slower/lower tone (doppler shift) but at no point did someone suggest that time was slowing down.
If a spacecraft left earth accelerating up to 80% of the speed of light and views earth through a telescope and notices both a red shift and an appearance of time slowing down (as you would expect), then on the return journey there is a blue shift and time appears to speed up (as you would expect) why does time actually HAVE to be tied to the blue/red shift effect? Why is it different than the hypothetical scenario with the sound waves? Furthermore the time's apparent slowing down on earth on the journey to the star is "made up" by the time speeding up on the return journey isn't it?