I have attended a couple of graphic design seminars for work, and I have always been impressed by the multimedia systems that the speakers used. They are demonstrating techniques to a large audience, so they have to have big equipment.
The speaker has a small computer hooked up to a huge display screen. The audience can see whatever he does on his computer enlarged several times for easy viewing.
He speaks into a small microphone attached to his clothing. The sound goes through large speakers so that everyone can hear him without straining.
I'm sure this type of multimedia computer system is quite expensive. They charge enough for attendance at these seminars that they can afford to pay for the equipment, though.