Can sound alter the DNA? Are the changes always positive, or can they be negative?

This is a simplistic description of what takes place. Overall, the answer is in the affirmative that sound can influence the makeup of DNA. This can have both positive and negative consequences, and that is something worth probing further because it will help to enlighten science and medicine about a number of phenomena that have been perplexing and inscrutable, and thus impervious to probing by existing scientific tools to gain a deeper understanding.