On some computers, there is a bug that they never bothered to fix. I have a three computers I use and one does not recognize any wav files in poser and these are wav files I know work since they work on another computer. Have you tried Okham's suggestion of putting a mic in the mic jack? I've heard from multiple people that this fixes the bug but have never tried it. If you don't have a mic you can actually use headphones and plug them into the mic jack. If nothing else, I'd be willing to try your wav file to see if it is a problem with the wav or if you're getting the bug.

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