Morality isn't something held by all humans but we can explain the desire for it in terms of evolutionary behavior, empathy, etc.  Just because there are sociopaths/moral skeptics/those who have different moral axioms does not mean there isn't some static first principle.  This static first principle would have to predict the realtivity of morality.  I believe such a principle is utilitarian in form, something along the lines of "a good action is an action which promotes average happiness".  Happinees must not be understand in the hedonistic sense, but rather in a sense that allows all beings to have different forms of happiness.  This would explain why different cultures have different moral codes, namely because different cultures have different people with different happinesses.

My proposal for happiness:

Person S is completely happy iff:

1) At time T Person S is in mental state M

2) Mental state M is the preferred mental state of S at time T.

We could then imagine humans and an alien species.  Our alien species may be closer to happiness at a concert if there is absolute silence, thus relative to our aliens it is morally proper to make the concert silent.  For our Humans they may be made closer to happiness at a concert if there is punk music playing, thus relative to our humans it is morally proper to make the concert have punk music.  Any mixing of these two groups will yield different results of morality for the action "playing punk music" in this circumstance.