"But perhaps it was only an echo." This "echo" may be the memory of music trying to touch him as Jonas is the keeper now of other memories. This memory of music may have been embedded in other memories. Hence, it is an "echo."
I don't think that there could be music behind them. Even if Jonas's escape changed his society so quickly, we have to remember that his community was far, far away. He and Gabe ran for so long that they left the community much too far behind to be hearing music from there. So I think it's an echo or maybe just something in their own minds.
Every year I have my class debate this issue (more specifically, the issue of whether or not they die). I have heard some really persuasive arguments both ways. I think to a certain extent there is evidence enough in the text to back either perspective. It just depends on which you find more persuasive, which might relate to what kind of person you are. Are you optimistic or not?
This should be moved to The Giver Discussion Forum so you can get interpretations from more viewpoints.
I think Jonas was hearing music from the village in front of them as he and Gabe come down the hill on their sled. Jonas had never been given the memory of music by the Giver, but the ability to recognize what the sounds were and their significance as a source of support, welcome, and love for the two hungry, cold, lonely boys was evident to Jonas.
Jonas imagined also hearing music behind him - the community using this joyous memory to help it deal with memories that were not so pleasant but that were also coming into their awareness as a result of Jonas's departure.