It is "But perhaps it was only an echo."
The context here is that Jonas and Gabe are heading down the hill on the sled that they found there. He can definitely hear music coming from in front of him -- from "Elsewhere." But he also thinks he can hear music from back in the direction he came from. But maybe it's just an echo.
So we are left with some ambiguity. How will Jonas's leaving affect the community? With all those memories set free and affecting them, will they be playing music? Or is it just an echo...