Does talking to babies help stimulate them?

Yes. In the second half of pregnacy some babies increase activity. Classic studies showed the fetus perceives vibroacoustic "sound" around 24wk. (seasoned nurses knew that for decades from accidentally making noises and watching fetal heart rate go up.) spending money on special music or sounds is not justified at this time. But having agreeable family members and mom "talk" to baby can be meaningful.
Yes. Early infancy is a productive time where babies brain acts like a sponge. It absorbs the rhythm and sing song quality of speech that allows them to speak later with the same qualities.