Recently, with some MP3s I bought from Amazon, I'd been running into this frustrating problem ("The entire file has been decoded to nothing but digital silence"):
http://forum.imgburn.com/index.php?showtopic=59&view=findpost&p=76706
I was unsuccessful in Googling around for the elusive v1.9.0.311 of l3codecx.ax; it doesn't even appear to come with the latest K-Lite Codec Mega Pack.
But it turns out that last month Microsoft released a security update because of a potential vulnerability in this codec. Installing this update installs v1.6.0.51 of l3codecx.ax, and after I installed it I no longer had problems with the Amazon MP3s.
If you have automatic updates on, you probably got this automatically on April 13. Otherwise, you can download the update manually here:
http://www.microsoft.com/downloads/details.aspx?familyid=B1582A74-4A7B-4540-BEB1-7C89C86EAE87&displaylang=en
Here is more info about the update:
http://support.microsoft.com/kb/977816
http://www.microsoft.com/technet/security/bulletin/MS10-026.mspx
