Hi Fred,
I am not sure if this diaphragm can be repaired... or if it would be worth the time...
I recommend that you buy a new speaker ([http://www.amazon.com/Gino-Replacing-Speaker-Receiver-Earpiece/dp/B007VBTYQM/ref=sr_1_5?ie=UTF8&qid=1366155609&sr=8-5&keywords=nokia+6230+speaker|here] or [http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=nokia+6230+speaker&LH_PrefLoc=1|here]) and follow [guide|3729|this guide].
Steffen