OK now that explains a lot. It says that the problem is not necessarily having to do with WMP or any of the codecs themselves, but instead probably a hardware issue with your drivers or a Vegas problem.
OK FYI Microsoft Sound Mapper is a built in feature of windows. I dont believe it ever needs to be replaced, only that there are occasionally problems with it.
Try going in Vegas to OPTIONS > Preferences > Audio Device Type and changing the Audi Device Type to "Windows Classic Wave driver" and click OK see if that changes it.