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I'm sorry to hear that, but you have to contact your Local support to get your device inspected.
On a side note, I don't know how you could increase the volume to a point were you damage the speaker unless you use third party apps
The phone itself as well as all the components are covered by warranty and should be fixed / replaced as far as you didnt damage it. I also dont understand how could the speaker be damaged by using high volume. You should send the phone to sony directly because the person in the shop doesn't seem a electronic expert if he actually said that.