Bônus Se tiver um microfone e pode lidar com chamadas do iPhone
Basicamente eu tenho um capacete de snowboard que tem alto-falantes embutidos e tem uma conexão de 3,5 mm (femininas) com os alto-falantes.
Bonus if it has a mic and can handle iPhone calls
Basically I have a snowboarding helmet that has speakers built into it and has a 3.5mm (female) connection to the speakers.
I can't vouch for it, but it looks like what you're asking for :-)
If you'd also want to skip the cable itself, I would suggest taking a look at the Jabra BT3030 Bluetooth "Headset", which you could connect with any male-to-male cable.
There are such on ebay.com, I saw one today searching for "4 conductor 3.5mm [extension] cable" - I agree, they are rare. (You might have to build it yourself by buying two iPod earphones and then soldering the four cables, but I hope that's not necessary!)