WMC is pretty limited when it comes to displaying subtitles. You might have luck with the shark codec pack. It is a collection of codecs. Search for it online and do some research. You might just need one tool out of it such as the lav splitter or one of the filters to get your subtitles working and a codec pack may be Overkill. I didn’t take the time to figure out exactly what I needed. I just went with the shark codec pack and played with the settings til I got it working. Hasn’t caused me any problems yet but I have had to update the codec pack quite regularly as it stops working after some windows updates or something.
Shark codec pack has two different sets of codecs. One for 32 bit apps and one for 64 bit apps. Wmc, I think is 64 bit and Windows Media player is 32 bit? I might have that backwards. Anyways, if you get the settings correct, when you play something in windows Media center, different codecs and utilities will start running depending on your settings in the codec pack. Icons will appear next to the clock on the task bar for them. While your video is playing in wmc, don’t have the window maximized so you can see the icons on the task bar. You might see ffmpeg running etc. One of them will allow you to switch between the different audio tracks, say if you have English and Spanish to select from and you can also choose from the different subtitles.