Unfortunately that's one thing I can't really test in anyway without an N900. This just rely's on pianobar to send the Audio which is likely using whatever the system is set at. In silent mode it's turning the sound off and thus muting pianobar.

There's probably a trick around that which is what the built-in media player does but I wouldn't have any idea how to do that and no way to poke around a system to find out. I can look through some docs here and stuff and see if they mention anything... but I wouldn't get my hopes up for that one for a bit.

As for the headphone bit... I'm sure that'll have some kind of signal in dbus. I'll have to look at the docs but is another one I can't test properly since the SDK dbus doesn't work right (and I wouldn't know how to "emulate" pulling the headphones out of the SDK.)