I use in-ear monitors. Better for my hearing, great sound, and it kills the roar while allowing regular traffic sounds.
Actually, somewhat counter-intuitively, in ear monitors are
not better for your hearing, at least not as compared to using
good earplugs and the helmet speakers, for a couple of reasons:
One is that the attenuation of the in-ear monitors, even the custom form fitted ones, is less than what you can get with properly fitted earplugs. Even foam disposable earplugs can deliver -30 to -33 dB (equivalent to 1000 to 2000:1) of attenuation. Ear monitors are lucky to deliver -20 dB (100:1) if they are rated at all. So right off the bat you are looking at having up to 10 times more road noise get through to your ear drums.
Yes, when you try to listen to the helmet speakers you do have to crank up the volume of the amplifiers to hear the music, but remember that the music is also being attenuated 1000:1 or more by the ear plugs, so the power level of the audio reaching your ear drum is small.
In the case of an in ear monitor the sound is being shot , and ducted by your ear canal, directly at your sensitive ear drum with no attenuation at all. Since the noise attenuation is less it actually takes a higher level of acoustic sound pressure delivered to the eardrum just to hear the audio at the same signal to noise ratio as when using the ear plugs and speakers.
There is no question that the in ear monitor delivers far better quality of sound. The distortion picked up by horribly over-driving the helmet speakers is considerable. Plus the earplugs do not attenuate all frequencies perfectly equally, so the sound fidelity suffers. But from strictly a hearing safety standpoint you are better protected by using earplugs and helmet speakers.
I already have some occupational hearing loss from back when I was young and foolish and didn't wear earplugs around the noisy machines we work on, so I try to protect what's left vs. getting the best in sound quality. The sound quality may be poor, but it can be entertaining at certain times. Other times I just listen to the bike. If I want to listen to some high fidelity music I do that in the living room, not on the bike.
As with most other things, YMMV