It would probably be advisable to keep the volume relatively low... High volume soundwaves can damage your hearing over extended periods of time (and yes, 30mins is easily long enough to cause permanent damage)
I personally keep my dosing volume to a comfortable level... Quiet enough to relax to, yet loud enough to make out all the sounds
I listen on my ipod and keep the volume 40-60%. You want a good signal over any noise, but you definitely want to avoid distortion so don't crank the volume too loud. Loud enough to clearly hear the white noise in the dose is loud enough. It isn't volume that tricks your brain into recreating the brain waves, it is just the the two brain hemispheres trying to sort out conflicting signals.
i find it varies by time of day and surroundings. At night in the dark when the house is still, i can get away with it at 30percent as well as others have said. but during the day i find i need the volume at about 80 percent to clock out ambience
I think my computer must have crappy speakers because I turn the volume all the way up on my computer and my i-doser player, and I still wish I could get it louder. Or maybe my ears suck.