Hi Sean
Are you using XP or an earlier (Me. 98) system, if it is XP then windows mixer is part of the OS and cannot be uninstalled, only modified by other programs, (The other programs disable or overwrite it in favour of there own) if you are using the earlier OS then it can be installed by going to the Add/Remove programs/Windows components/Multimedia.
If you think Media player is the problem then download the latest version from Microsoft and install this. (It is possible the update corrupted some files)
Another thing you can try is to run Scandisk (Choose to Automatically fix errors) and see if this solves the problem.
Hope this helps

Bill