For the record. I have been using my PS3 to play audio recorded from WMP to separate USD drive. Initially I could not see the files, but there is a way to do it.
I found this extremely helpful:
November 10, 2006 - In our first look at the PlayStation 3's media playback features, we initially the system to be limited by its directory structure design -- unless media was stored in the proper folder, it would not be read by the PS3. But surprise! With a tap of the Triangle button, the PS3 becomes a fully-formed directory searcher, and can indeed find and play music, photos, videos or playlists from anywhere on most any media device.
The PS3 itself is designed to browse only the folders it is designed to browse -- the root-level VIDEO, MUSIC, and PHOTO folders (as well as the standard MP_ROOT and DCIM folders shared across many media devices.) It will load media directly from the system and load them onto the PS3 menu in the proper media setting, so if you choose Photo on the PS3 menu, it will show the images in your PSP or Memory Stick or Compact Flash or SD card or USB keychain folder. More importantly, it will not show media not in those proper folders -- unlike on a PC, it does not have a "media search" function to find multimedia elsewhere on your external device.
But never fear, because there is also a "Display All" feature when selecting your Memory Stick or other USB-connected device that allows you to browse the entire directory structure, and thus load whatever's on the drive that the system recognizes. This indeed works for any media type. We were able to run music from an iPod and pull video from a random spot on our USB mini-drive. It's a hidden option that some may not notice at first, and sorting through folders isn't exactly easy (an iPod is full of all kinds of dummy files that are not music, for instance, and the folders are not named after the albums or songs stored in them -- if you can sort your media for proper playback on PS3, you're going to want to), but at least the option is there. Thank the Lloyd!