Flip4Mac should work for you. And, it's handy to have the upgraded version for importing or converting wmv to other formats.
I suggest you post a link to something that is not now playing back ok. I (and others) can see how it does on my Mac, and report back.
Since WMP is no longer supported for Mac by MS, F4M is the preferred method. However, there
is a detail within the F4M preferences that allows you to choose whether to use it exclusively, or have the option of using WMP. It is located under the "browser" pref tab, and says "use Flip4Mac WMV browser plug-in". When you do not have WMP installed, this option is grayed out and not selectable, but with it installed, you should have the option to uncheck it.
One other place to get advise is from the F4M user forum
HERE