Maybe the stereo installation caused a drain and yes the battery may be dead. Well, that's my bet anyway.
I have a 4/2009 build 335d and the battery is still good. But I live in California, so YMMV because colder climates may be harder on batteries.
I do not have CA either, maybe that drains batteries too.
Anyway, take it to a shop that knows BMWs to check the battery because if they do end up replacing it they need to register the new battery with the car or it might blow up.