The eyes are stimulated at different frequencies as well. Visuals refer to the shifting, colored patterns perceived when you look at the flickering lights on a light sound system. The patterns are optical illusions caused by the interaction of the flicker rate with the sensing and processing rate of your eyes and brain. You may have seen a child's top with radial black lines, that when spun, gives the appearance of changing colors. That is a similar effect.
Volume refers to the loudness of the sound stimulation.
The above referenced material is from the Mind Machine Buyer's Guide, courtesy of Neuro-Technology Research. Medical Disclaimer - please read.