You know what, now that you mention it... this sounds a LOT like the problem my roommate experienced in his G. He switched to AM/FM thinking his speakers might be screwed, but it worked perfectly. The next day, he could not replicate the problem.
I chalk it up as Nissan's horrible way of trying to bring the speed-sensitive volume control to their cars. I know that system works flawlessly in Pontiacs, and I can't stand it. I think you should be able to adjust the volume yourself for the conditions. If you have the windows down you're going to have the volume up. Windows up, volume down. What happens when you have the windows up and you speed up? It'll just get unnecessarily loud! (Just speaking from experience riding in my buddy's Grand Am).