hi, i recently had the exact same problem.
i figured the cause of mine to be the installation of an audio/video software: VLC. although i've uninstalled it, the problem still persisted.
i then did a system restore to a date prior to the installation of said program. and voila! it worked 
however, if your problem has been around since purchase, then i suggest you take it back to the agent or send it for service (this should still be covered under warranty).
if the problem has only surfaced after a certain date (probably due to an installation of a new audio software, adjustment of association files, windows update, etc.), then maybe you could restore your system to a date prior to it.
hope this helps! 