I just did this on the weekend to my Arai.
Picked up a pair of JVC HA-S160-V and disassembled it. The speakers were about the size of a quarter if not smaller.
I took out the cheek pad and pulled aside the fabric. The Arai check pads have spacing for your ears without foam, so i stuck the speakers there with double-sided tape. Much like the picture below.
Pulled the covering fabric back over it and placed it back in helmet. I didn't lose much volume cause i removed all the headphone padding and just used the cheek pad fabric