I think one interesting question is if there is a difference between morality and ethics. I generally interpret ethics to be social rules (formal or informal) about proper behavior/action. Morality tends to get wrapped up with some sort of worldview, generally religious. Ethics are relative to the social group in which they form. Most people, I think, view morals as absolutes and universal (though there are few of these that are, in fact, evidenced in cross-cultural comparison).
For myself, in everyday practice, I don't worry too much about ethics vs. morals, but just consider the path of most harmonious action. That is, in all cases, I try to intuit what I should be doing. This is not always consistent superficially, though I have found some absolutes for at least myself, which I suppose you could call morals.
Rather than think in terms of rules, I have very few rules for my behavior. I try to work toward the ideal of love, and I think in terms of Most Harmony. What is best in my situation? These are my questions.
What is / are the source /s of it?
I believe we have the intuitive power to know what is most harmonious, what is the best action, because we are a part of God. God is All and Beyond All, and is perfect and whole. So when we recognize and cultivate God's spirit within us, our actions align with what should be and there is harmony between us and everything else (which is really to say, there is a harmonious function of the All).
Societies determine laws and ethics and norms, but I think these partially grow out of that inner voice and partly are warped to meet people's self-centered needs. So while I recognize society is a source of ethics, I don't think it is the only source or even the best one.
How is it maintained in a society / Why does it deteriorate? (by what measures?)
Maintained in society through informal sanctions (gossip, relationships, etc.) and formal ones (laws, courts, police, etc.). I'm confused as to what deteriorates. This could mean a lot of things. Ethics can change because society changes. Or people could stop following social norms and ethics as much because there is a lack of solidarity (generally tied to anonymity and alienation from others). I'm not sure if you mean why ethical codes themselves change or fall apart (when they fall apart, they often do so as a culture falls apart) or why the general populace starts failing miserably at acting in accordance to ethics (i.e, the gulf between ideal culture and real culture widens).
I think ultimately, morality in individuals fails when individuals are centered on themselves to the detriment of others, and thus they do not cultivate that which they really are (a part of God). When we stop listening long enough to that small, still voice within, eventually we forget how to do so. It's always there, but we have to embrace it to grow in it.
A handy social mechanism for teaching and enforcing moral/ethical rules.
Not necessary, but useful.