Stupid question: How loud should I put the music on my iPad for my unborn baby to hear?

I know the ears are developing still, but I'm not sure how loud or low the music should be. I don't always use headphones. Sometimes I put my iPad on airplane mode and play classical music with the speaker to my belly. 
I'm just trying to learn and prepare, no question is a stupid question when you are a first time mom. No rude comments. TIA! 🤗