Take the case apart following this guide: [guide|4333]
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Take the case apart following this guide: http://www.ifixit.com/Guide/Repair/Installing-iPhone-4-Speaker-Enclosure/4360/8
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When you get to step 20 clean the contacts for the speaker and mic.and reseat them and re-assemble.
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When you get to step 24 clean all the contacts for the speaker, ensure there is no obstructions, blow all the dust out (there will be a lot) then reseat them and re-assemble.
Take the case apart following this guide: [guide|4333]
When you get to step 20 clean the contacts for the speaker and mic. and reseat them and re-assemble.