I did blow three mid-drivers when I was hosting a party and we were, ahem, performing karaoke over them. My wife still laughs at the thought of seeing smoke puff from the dust caps mid-peformance. I returned them to Gallo, which confirmed that they were really fried. We were not singing at high levels.
That is quite normal for consumer speakers and not just Gallo. There are a great deal of dynamics in the realistic reproduction of the human voice from an amplified microphone.
If you are interested Rock Band is amazing - much more fun than simple karaoke because those people who are not blessed with a good voice still get to contribute...and there is more opportunity for self expression.
I also suspect there are some well designed limiters on the USB microphone used on Rockband - to avoid distressing speakers with heavily clipped signals.