Let me make one thing clear: I am fully aware that the change in timbre comes from the change in acoustic wave speed when going through a different medium (just like light).
However, would I not need to have helium in my ears to hear the change in timbre? The medium in my ears is air, so the timbre should go back to normal once it leaves the helium boundary, right? Say if light goes from glass to air it will speed up, but it will slow right back down as soon as it goes back into glass again, so if I was in the glass, I would never experience the speed of it in air. Why do acoustic waves seem to not have this behavior?