Wow, Gary, great post! I had never heard of either of the above mentioned items. I did not take time to read through the whole explanation of the Valsalva Maneuver just yet, as it is highly detailed and it is getting late. I need to read through it when I'm fully alert to get the max benefit from it. I thought they said that power lifters use the technique, though.
My bet is that the second thing you proposed, respiratory alkalosis, is the correct guess as to what happened to him, as trying to sing hard is exactly the activity he was engaged in.
Unbelievable that someone on such a relatively small forum would know the answer to this. Thanks again.
Tim