It's an ambiguous ending.
The immediate description is that Jonas has found another pocket of civilization -- and that this one is characterized by joy, music, singing. He and Gabe slide down through the snow to them, in the hopes that they will reach the community on time, be taken in, and nursed back to health.
However, in Jonas's weakened state, this vision is not entirely reliable -- and elements of the paragraph make him less reliable yet. The "sense that he heard music behind him" -- probably impossible -- "but perhaps it was only an echo" is a fairly strong implication that Jonas cannot really tell what he is seeing or hearing. Similarly, his sudden certainty that "they were waiting for him; and they were waiting, too, for the baby" is obviously not a rational one -- even if it is true, Jonas has no way of knowing it.
So a lot of Jonas's certainty and excitement here are entirely unreliable. It's possible that he does see, or hear, something, and is responding to it deliriously. It's also possible that there's nothing there, and this is Jonas's final, comforting delusion immediately before collapse.
So, it's ambiguous. Either they found a haven, or they died; you don't know for certain. Which you consider more likely is your own interpretation...
Eventually Lowry wrote additional books in the series, where Jonas has clearly lived. Gathering Blue (published in 2000) only gives an ambiguous hint that Jonas has survived and arrived into the town where the sequel takes place; but in Messenger (2004) and Son (2012) this is made explicit, and Jonas appears as a speaker character. However, I believe The Giver was originally written as a standalone book (the next one was written seven years later), and certainly stands as one. So, whether or not the sequels "determine" the meaning of the ending of the book, is also up to you :)