The noises of your heartbeat, blood, and digestion, plus the echo of you speaking makes the womb super noisy. One study I read actually said that it's as loud as a vacuum cleaner constantly running, so the baby really can't hear much outside of you. The reason they can recognize mama's voice is because it echoes inside of you. Everything else is just background noise. Even if you played music super loud, your baby won't be able to really hear it well.
Your phone isnt going to play music that is too loud for your baby even all the way turned up and laying on your belly. There are lots of layers that sound has to travel thru to get to baby. Everything your baby hears is very much muffled.