It is important to realise how the ending of this excellent novel is completely ambivalent: Lowry seems to not want to give any clear indication either way as to whether Jonas and Gabe survive or not. Note how they are absolutely exhausted and cold, and as Jonas starts on the final sledge ride he feels himself "losing consciousness." As they go down on the sledge to "the Elsewhere that held their future and their past," the fact that Jonas had always felt this place waiting indicates that it could be a possible afterlife. Certainly the hearing of music at the end suggests some kind of supernatural destination, which could be what happens to them after their death.
However, at the same time, the destination is also described in concrete terms:
He forced his eyes open as they went downwards, downwards, sliding, and all at once he could see lights, and he recognised them now. He knew they were shining through the windows of rooms, that they were the red, blue and yellow lights that twinkled from trees in places where families created and kept memories, where they celebrated love.
Jonas feels sure, from this quote, that they are headed to a concrete destination, where, unlike the community he has just fled, memories are kept and cherished, and where love exists. The way that Jonas hears music from "behind him," from "the place he had left," could suggest a happy ending for the community to, that somehow Jonas and Gabe are able to transform their home into the kind of place where they are heading to.
At the end of the day, however, you need to take your pick about what you think happens at the end of this story. The author provides us with enough evidence to suggest both endings. What do you think?