DLL Reference in unity

I want to use WMPLib to play mp3 files on runtime. i added it in Visual Studio Solution and there are no errors, but in unity it says that WMPLib is not found

Did you put it in a Plugins folder ?

As _met44 implied, you need to actually add the .dll to your project. Note that for regular c# .dll’s (which I think this is?) you do not need to add it to a Plugins folder; any regular folder will do. The plugin folder is only required for libraries of native code, as mentioned on this page: Unity - Plugins.