Domio Sports makes a device that turns your helmet into a speaker itself:
https://domiosports.com

Terrano Systems has speakers that "hover" over your ears

https://www.terranosystems.com

Afterskokz makes a bone conduction headset that works well with bike helmets
https://aftershokz.com/collections/h...ucts/trekz-air

All support bluetooth and can also be used as a headset to a phone. Domio is brand new but the other two have been around for a while and I've used both of them. They all keep your hearing open to the ambient sound for situational awareness.

J.