Well i bought the K2 Rival pro in L/XL, audio wasnt the loudest but im not exactly after having my ears blown off! was loud enough that i had to stop the music to hear people talking loudly at me in the shop, probably going to suffice on the mountain!
The helmet was without doubt the most comfortable helmet i tried on for my head size, the lining inside was just so soft and snug! Built in audio with it was just an added bonus! Will let you know how they hold up. if the audio sucks or break while im on the mountain i'll let ya'll know. If that happens i'll try a conversion job with the Koss too
Try looking at some motorcycle sites that sell audio equipment for helmets, got some three years ago and they still work great. I think they were around $80, don't remember the brand but ill post it when I dig my shit out.
I bought the RED Hifi and ordered the RED headphones to drop in. Love the helmet but the headphones are garbage.
Then my buddy got a RED Frends edition that had the audio built in, I tried it on and it was 10x better than mine. So if you get one of the RED Frends version they are legit. You could also try to find just the Frends audio part separately but I had no luck in doing that.
I just put a pair of regular Phillips earphones into the audio pads of my Giro Revolver helmet.(Not the in-ear buds, the ones that just "sit" in the ear) It sounds fine, but I have to wear it so the cable is behind my head because the two end cords aren't long enough to go over my face. And they don't have a mute button on the cable but I plan on getting a pair that does.