iPhone 5 video playback no sound

My iphone 5 is unable to play the sound which i recorded using video camera.

I didn't use any headset during the recording but when i play it back, no sound coming up.

I tried restoring as new phone but still unable.

I bought my handset in singapore and it's still under warranty period.

But, i've lost the receipt, can i still claim the warranty?

 

Another issue is about the personal hotospot.

Is it true that ios 6.1.x unable to tether your mobile plan via wifi?

Im using blackberry 9700 as my other phone anyway.

 

Thanks for any help.

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