Help, Sound problem

On my nephew's iphone, his sound does not work in some situations; the music, the sounds that the iPhone generally makes during texting, and you're unable to hear or speak in calls (The caller can't hear you as well). BUT, when he receives an incoming call, you can hear the ringtone, you can also preview selected ringtones and when you put the phone on speaker during a call, you can hear and speak into the phone clearly. I tried restoring the phone to see if it fixes the problem but it doesn't. Does anyone know what the problem is or a solution to it?

Thanks

these are fixes people found

Try pluging in earphones and playing music then take them out sometimes resolves this problem.
A few people have posted this problem, others have solved by restoring try holding the home button and plugging in usb, if all else fails you can try restoring the bb by flashing the fls and eep files, search for posts on virgin restore. This will require you to unlock all over again.
Good luck

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this is what I did.. placed phone in flight mode went to system sound and started tapping different sounds for the ringer went back to ipod played some files and slowly the volume started becoming audible again.. its been 2 weeks since i have been able to hear anything but a slight souund ..haleluuuyah!!!! hope this helps others!!

Thanks a lot gaz! Plugging in the headphones worked like a charm!