HCAAdmin,  is this a streamed problem, if the file has no audio but the video plays fine  all I can think is is make sure the video is fully downloaded.
Does WMP throw any errors?
The site that gives you this problem is it using it's own portable media player maybe switching between videos the setting change in the player sometimes they kind of default to small vol settings to protect the viewer from getting blasted with next video.
If the video are submitted by users anything is possible.
Make sure all your settings are correct and none have changed.
Your system meets all the requirements for diffferent podcasts.

This from Wilkie
Receiving and using podcasts
Making full use of podcasts' syndication features requires appropriate software, often referred to as a podcatching client.
The feeds are usually distributed using RSS or Atom protocols to the podcatching client.
The dominant podcatching client is Apple's iTunes player.
 However, there are alternatives, including Microsoft's Zune Marketplace, Juice (multiplatform), Doppler (Windows), Podget (Linux) and Podracer (Linux).
Some established audio players, such as Amarok, Winamp and Mediamonkey also offer (sometimes limited) podcatching functionality.

Many podcasts also allow users to direct download, by giving a link to the audio file in an RSS feed or web page.

Podcast listeners can listen in one of three ways: through an MP3 player, on a computer using media player software, or with VoIP technology by calling to a virtual phone number.
By dialing a phone number, you can hear a menu of available podcasts and features.
 Find more about it in the external links section.
http://en.wikipedia.org/wiki/Podcast

On this note can't really offer any ideas why you have problems with "some" audio
 if the problem is not the actual podcast.
Have someone else test playing the same podcast would be a sure fire resolve
and determin if the problem exists in your system or the podcast itself.

Cheers

Thankyou HCAAdmin good to see you have audio again,
with WMEncoder there is only version 9 that I know of.
Cheers Mate