recording sound fades away after few seconds

[Hansie]

Hi,
First of all sorry for mu poor English.
We bought a second hand camera, Panasonic NV-DS29EG.
It works perfect but we have a problem with the sound. Because the recording sound fades away after a few seconds. Is this just an ajustment or a failure? I hope someone knows the answer as this is her birthday present and she is a bit disappointment at this moment.
Thanks in advance!

Greetings,
Hansie