I have never seen speakers in a helmet and they would also be heavy*
I’ve still got a Mace DJ helmet with some built in speakers embedded by the temples, there’s a 3.5mm socket positioned at the back of your neck.
TBH the speakers are shit and one stopped working after a while, but it only cost a quid more than the version without so I thought I’d try them out…
When they were working they did at least mean I could still hear my surroundings with a sort of tinny backing track, nice idea, not actually sure i’d want it in a FF though, plus I think others can faintly hear your music, and I felt a bit anti-social imposing my dodgy musical tastes on other trail users.