Sound under multiple user accounts?

I am a 3 week old switcher and am using my iMac as the family PC. I often have 3 user accounts active at one time, one for each member of the family. I noticed that often I can get any sound out of iTunes in my window because my daughter might have a webpage open with some music playing in it (flash game). Flipside, she does not have sound if I have iTunes open. What can I do so that she can have sound in her windows and I can have it on mine.

Also, not sure if I should make a new thread on this, but can someone tell me how to make the desktop icons and text smaller? I would like to fit more text on my screen if possible.

Thanks!