I've helped a few people over the past while when they can't see/hear a file. Either video or audio. And sicne I just typed this up for someone with all neat and tidy links, I thought I'd go ahead and make a post about it and label it as tutorial so now I can just point to it from now on.
I'm coming from a PC view point, never have worked on a Mac, and I don't know if it makes a difference, but one of them should solve it. I'm not an expert or anything, and everything's been trial and error for me, so you might have to work several things to find your particular fix.
The first one is to download WinAmp if you don't already have it. It's a great program, and it plays every single filetype there is. It's the best invention ever, I think. LOL. Download it
here.
The second solution is to download the K-Lite codec pack. If you want to read about it, go
here, but if you just want to download it go
here, and I'd download either the Standard or the Full. Then, bring up the media player (I think it's actually just called Media Player) and open the episode through that.
If K-Lite doesn't fix it, go
here and download the codec pack all-in-one. It might make you uninstall K-Lite, I forget. But really... either that or K-Lite should fix it.
As a last ditch effort? Try VLC Player. It's kind of a crappy player, and I use it as a last resort (it plays the sound and picture fine, but the sound seems to always be muted, and there's static, and you have to go to the audio menu and select "club" to make it sound decent) but it plays most things, so. It won't be, ya know, dolby sound and wonderfulness, but it plays well enough. Download that
here.