Are you finding sdl_mixer.dll just press download button to get your sdl_mixer.dll file.
If some software in your windows OS wants sdl_mixer.dll your operating system will ask for DLL file. When that DLL file is not available you will get this error.
If you have moved your file or changed the name of dll file you will get this error.
You will mostly see there errors when you dll file is missing:
If you need sdl_mixer.dll Download it right now and install it into your system to fix the error as soon as possible. just Install sdl_mixer.dll and fix your error.