Don't worry about apologizing for every missed kill or fight. It only serves to disrupt your cast even further. There are 10 people to watch as a caster. Because of that, you are not going to see every fight, and that's just how it is when watching dota games. But instead of taking time out of your cast to apologize for every missed piece of action, fill that time with more analysis or predictions or statistics checks like cs scores or something. Don't waste time focusing on the mistakes! Focus on the game at all times! It will improve your casting a lot.