While I understand that Maemo OS is billed as open source that may not mean that all other components and apps on top of Maemo will also be open sourced.

So the Music player not being open sourced is highly likely and does not conflict with the fact that Maemo is open sourced.

If I develop an application for Maemo, I can close it down and not have it to be totally open. The Music player just happens to be integrated and built in. It need not be also open as the OS underneath.

Google has built up its infrastructure and its search universe on open source too, but that doesn't mean Google's code is all open sourced. No way.