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