These are definitely an interesting product. Full information from their website:
Introducing bellybuds®, a safe and convenient option for prenatal bonding with your baby.
Our custom bellyphones® gently adhere to the belly and play directly into the womb, so your developing baby can hear prenatal music, voices, soothing sounds, or whatever you like.
bellybuds have a fixed safe maximum volume so you won't have to worry about turning them up too loud.
Skin-safe, hydrogel adhesive rings secure the bellybuds to the unique and changing curves of a pregnant belly.
bellybuds go directly on the skin and are easily worn under your clothing. And because the sound is dampened by the buddon covers, you may freely play music anywhere, anytime without disturbing those around you!