Descobri que eu deveria estar escrevendo valores assinados como explicado por roaima :
There's very little documentation on this that I can find. However the one example I can find of accessing it uses a variation on a signed value, such that values exceeding 172 need to have 256 subtracted from them. In your case, the value 226 should be represented as -30. No, I don't know why.
Escrevendo -30 -30
woks:
# echo "-30 -30" > /sys/kernel/sound_control/headphone_gain
# cat /sys/kernel/sound_control/headphone_gain
226 226