Not only do you need to see the presentation, but you need to be able to hear it as well. To reduce initial costs, the speakers supplied with whiteboards are not usually adequate to be heard over a working classroom level of noise.
The classroom / board room size, the quantity of people in the room, and the amount of conversation you feel comfortable working alongside are all important factors. People absorb sound. But of course, a fine balance needs to be determined because if the whiteboard speaker system is too powerful, it's a waste of money!