My husband says he study the difference in the type of music to play for a baby in the womb. He goes to the Pandora app and he only play classical music which is instrumental and he reads to our baby.
I guess that depends on what you mean by "effective." I try to spend as much time as possible with music playing nearby. My son kicked and punched and did flips when I went out while pregnant to the club my husband DJs at, I always figured he liked it.
My son is smart but I don't attribute that to music playing in utero. He loves music and singing and playing instruments but he's heard it every day since he was born, not just in utero.
I don't know what you mean by "does it work" - it helps me to have music on, it keeps my stress level down, therefore it helps me. But the effect of music on a person can't be quantified. There's no "effectiveness" measure.
My bf played the guitar for our daughter and she loves music and dances all the time shes almost 3. She would move when I was pregnant with her so I think its effective. He is just starting to do it again now that im almost 24 weeks