don't get upset but I'm a total layperson and I'm not really understanding the twin paradox and I'm just wondering: If the twin who left earth in a rocket so that he wouldn't age as fast had a walkie talkie of some kind to contact the twin on earth with, would the twin on earth hear the spaceship twin's voice talking veeeeeeery slooooowly, and would the rocket ship twin hear the other twins voice squeaky and sped up?
Simplisticly neglecting the workings of the comms device you are right, yes. How noticeable it would or would not be - depends simply on how fast the relative velocity is.
(For an actual walkie-talkie range would be a problem and the signal would be red-shifted, so this would have to be compensated for before you could observe the sound change).
Rather like Doppler shift of sound waves in air at low speed.
You would see the effect with a remote camera too, btw.
If there is a telephonic communication (say, with the help of EM waves) then both of them will experience that (during the outward journey) the voice of their brother is slowed down in pace as compared to their normal way of speaking. It won't be that one experiences it to be sped up and the other will experience it to be slowed down. The situation is completely symmetrical between the two brothers as long as the spaceship brother doesn't take a turn (during which he will experience an acceleration and will break the symmetry between the two brothers). How much the speaking pace will be slowed down will be dependent upon the relative speed and will be owing to the time-dilation and as well as because of the doppler effect.