As I understand it, Musiconomi is built on top of Musicoin, basically a token of Musicoin, though to pay for their ICO its in ETH. So though the two projects are not related in a technical business sense, they are technically intertwined by the token. The Musiconomi team formerly was part of Musicoin. As for Opus, they are part of the Musicoin Alliance so they are working together and Opus is looking into integrating the Musicoin blockchain as described in this article.