Windows Media Player is super finicky. There is suppose to be this priority system where the codecs on your system report a value representing how likely they are to be able to read/decode a specific format. But developers got lazy and just started marking everything as the maximum value. So Windows just randomly chooses a one based one an obscure ID used when installing the filter.
So in your case it's likely not working because something on your system installed a filter which is taking priority but isn’t actually able to decode the file. The pack didn’t work because that other broken filter still has priority.