I don't know, really. This doesn't seem like a feature I would ever use. Like if I'm playing an atmospheric game like Last of Us, why would I want my own music blaring in the background? Or if I want to hear my own music when playing something less immersive (ie: Call of Duty) I could always just turn on my own stereo...
Yea I really don't see why people are so pissed. This is getting to xbox one 24 hour online checks level of bitching. The no external hard drive thing sucks for sure. But I'm not about to cancel my preorder over not being able to play mp3s while gaming. Who does that so much that its a make or break feature?