MS has a patent on OS based in game music, meaning ALL games can play custom music in game and the dev doesn't really have to do anything to enable this feature.
To get around this, the dev for the game has to implement the option themselves. Just because you can access in game music via the XMB on warhawk doesnt mean Sony can automatically make all games support it.
The option of accessing music via the XMB isn't OS based. All its there for is so that devs don't have to create their own interfaces to enable it.
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