In my opinion, at the end of The Giver Jonas and Gabriel just fainted and before he closed his eyes he had memories that were getting recalled.
The reason why I strongly believe that Jonas was just remembering his past is because before Jonas closed his eyes, Jonas noticed the houses in the other community he reached to. When he went on the sled with Gabriel he perceived the memories he experienced during the training with the giver. That moment just returned the memories he had. Another moment they had before they fainted was when he saw the houses filled with people singing, laughing, enjoying. I am assuming that Jonas just had a theory and the voices simply echoed in his mind due to the state he was in. The reason why they passed out is probably because of their weakness. They didn't get a good source of good and warmth and were very tired of travelling form one community to another. In conclusion, Jonas and Gabriel just passed out and the people on the other community will probably take care of them and give them the warmth the other community didn't. Hope this helps!
