middle button acts like a mute button, and also serves as a play button when using the media player. You only have one option for a convenience key, because the 9900/9930 only has the one. Located on the lower right of the phone.
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You can't. The only convenience key you can set is the one on the lower right side. You access it through Options > Device > Convenience Key. The middle key between the volume controls is hard-coded into the OS as the Media Play/Pause key.