1) FuunctionNode: msg.url = msg.originalMessage.voice.file_id;
return msg;

2) HttpRequestNode: https://api.telegram.org/bot<TELEGRAM_BOT_ID>/getFile?file_id={{url}}

3) FuunctionNode:
msg.path = msg.payload.result.file_path;
msg.payload = "https://api.telegram.org/file/bot<TELEGRAM_BOT_ID>/voice/"+msg.path.substring(6, msg.path.length);
return msg;