This could possibly happen owing to audio driver problems. To eliminate possibility of any driver related issues please uninstall and then re-install the audio drivers. Click here to download the updated audio drivers for Dell Inspiron 6400.
Please let us know the status after updating the drivers.
To know more about Dell's products, services and drivers & downloads, please go to http://dell.to/YBlak6 Thanks & Regards
Zohaib R
#iworkfordell