I believe in the Mozart effect. Fetuses can hear and listen as early as 16th weeks, and by playing happy classical tunes will stimulate brain development. Why Mozart? I guess that's because his works are mostly cheerful.

A friend of mine shared that her firstborn was not as observational as her 2nd child. During her 2nd pregnancy, she listened to alot of classical songs, she believes that's the reason which induced the brain development in her 2nd child.

Chanting or saying prayers may calm your baby too, if you believe it works. But your baby definitely can recognize your voice and your DH's voice so do talk to him or her always and sing/ hum to him/her. It is afterall the special bonding between mommy and baby.