I'm sorry, but glue will not fix this problem. When the rubber or foam edge of a speaker breaks down or deteriorates because of age, it becomes almost like dust, which is why it cracks and then breaks apart and falls away from the speaker. The only way to fix it (besides replacing the speaker) is to replace the edge, or "surround". At this link, http://www.simplyspeakers.com/All-Foam-Kits_c_118-1-3.html
, you will find a kit that you can buy to repair your speaker for about $30. The standard 12" repair kit will work if you have an 'angle cone edge attach' type speaker (see the bottom of the page for a picture of what this means). This page, http://www.simplyspeakers.com/speaker-repair-foam-edge-replacement-instructions.html
, will show you step by step how to use the kit to repair your speaker.
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Thank you for the opportunity to help you.