That's a fine question.... putting it in the kernel overlay code makes config.ini options easy. eg, overlay_vol_min= { 0 <= int <= 100}, overlay_can_unmute={ 0 | 1}, etc., and at the moment I can only see benefits to doing that - we'd be removing the need for an app developer to 'get it right'.

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I should have hit refresh before posting
Not sure about when pcmplay returns, but that certainly would be another good reason