My search is finally complete for a set of earbuds for snowboarding that
1. do not noise-isolate, so you remain aware of your surroundings
2. stay in your ear without the need for constant readjustment
3. comfortable with decent sound quality (as decent as earbuds can be)
4. compact and fit under helmet / balaclava
5. do not have the stethoscope effect that often plagues in-ear canal earphones (this effect transmits fabric rubbing and snowboard ground noise into your ears)
The lucky winners are Yurbuds and also a similar no-name product from Philips called "in-ear sports headphones." Both use a grippy silicone disc that fits over standard earbuds. That's how they stay in while you ride.
Yurbuds.com & Yurbuds review:
Yurbuds Ironman Review | Anything But iPod
Philips ($19 at Target)
Amazon.com: Philips Shq1000/28 Sport In-Ear Headphones: Sports & Outdoors
I wonder how these discs would do with quality Yuin PK3 earbuds..