I tried some helmet speakers in my kids helmets this past year that we purchased on Amazon----they did not work very well at all but they were cheap and you get what you pay for
my kids race miniwedges (go carts with a roll cage) and a helmet speaker system would really help me out being I would have one less thing to plug in and keep track of----
I investigated the concept at PRI and hooked up with a guy that professionally installs quality helmet speakers
his name is Steve Thomas and his email is steve@stevethomasracing.com he came recommended by the people from Bell Helmets
I am sure its not cheap but this guy is the real deal.
I feel confident his system will work for my class of racing as the noise level is probably 10% of what it is in a big car ---not sure about how it would work for full size cars with much louder conditions
Brad