You can talk freely with all helmet walkie talkies. The ear pieces fit nicely inside the ear cups of the helmet. The only problem is, the mic has to be touching your bottom lip or it won't pic up the sound (this is so you're not listening to the engine of the sled constantly). Therefore, you probably wouldn't want to share your mic with too many people due to the slobber they'll leave behind.
If you buy the HJC Chatterbox you can do exactly what you want. It mounts to your helmet. Most HJC helmets have pockets inside for the earpiece. The mic mounts in the mouth area with velcro. It doesn't have to touch your lips like one of the guys said. The mics are voice activated so you don't have any buttons to press. There is a sensitivity adjustment so that you don't always key the mic when you hit the throttle. I've had mine for about 5 years and I would never ride without one again. They cost about $240 here in Canada but they are worth every penny. No more stopping to talk to your buddy about where you are going or where to turn or if you need gas, etc. They are also a great safety feature as the lead rider can warn of dangers on the trail and approaching riders etc. Make sure you buy a spare battery. The battery will normally last about a day and a half of riding, depending how much you talk. The HJC is the best I've seen yet due to the fact that everything is contained in/on your helmet so everytime you take your helmet off there is nothing wired into your coat or sled. If you do a search in this forum for chatterbox you will probably find some extensive posts we had about these a year or two ago.
I don't know what the latest Collet radios are like but my buddy has an older one. It sounds good but it has a battery pack ( that is actually fairly large, it may also have some of the radio electronics )that you keep in your coat, so there is a wire running from your helmet that you have to disconnect every time you take your helmet and/or coat off. It may not seem like a big deal but if someone makes one without a cord I can't imagine dealing with a cord. One advantage the collet may have is longer life between battery changes. I bought a spare battery for my Chatterbox and always carry it with me. I can usually make it through the weekend with the two batteries. I take the power cord to charge it if I am going longer than the weekend.