HERE is a good place to start. Basically, at birth an infants hearing is, on average, very close to that of an adult, on average. I would certainly start on the quiet side and also not play music with huge dynamic peaks. If you can hear it then it is highly probable that the child can hear it. I certainly would not claim that playing classical music will make one smarter but it also is likely to have SOME effect. There are actually quite a few studies in the primary literature. When our son was born I did do a bit of research.