hey spanner,
Kate & I use 2 different versions of noise cancelling ear phones in our helmets whilst riding. Kate has an R-Jays FF NFOT (full face, non-flippy-open type) & I have a Shoei FF NFOT helmet. We use dual Sena 20S units so that we can talk to each other, listen to music or make/recieve phone calls via bluetooth.
I use the Sena in-helmet speakers as well as foam ear plugs (the same type I wear at work everyday). I first tried using just a set of in-ear headphones, plugged into my 20S unit, but they made my ear canal sore after an hour or so. For me, the speakers/ear plugs combo does the job very well.
Kate uses Philips in-ear headphones ($12 BigW) plugged into her 20S unit. She has created a pair of moulded, in-ear, earplugs using mouldable silicone putty which adheres to anything, sets to any moulded shape and can be blended to create different colours. Cheers. John & Kate.