I was curious as well so I did some digging. Apparently it's quite an involved process. First, you have to get both the video and the audio file from the *.usm via demuxing it. Next you have to convert both of them to an easily playable format like MP4 and M2A. Finally, you have to mux them into a single MKV file which you can then play normally. I'll try to make a step by step tutorial later but I've tried it and it works.Here it is.