The Closed Captioning For Silverlight Media page at T2Sami.com says in its commentary:
A number of very sophisticated Silverlight Video players are under development: some open source, some proprietary…. The commitment to captioning in Silverlight is not so well supported. If it is included at all, it is frequently an afterthought.
In my initial series on HyperVideo I started with the media player emitted by Encoder, and stripped out the Closed Captioning button. That was unnecessary and I regret it. Please take this posting as my pledge not to have closed captioning and other accessibility issues be an afterthought; but rather they will be an integral part of the spec I will deliver for HyperVideo on December 1 for the renewed HyperVideo project.
In the interim, please take a look at this excellent tutorial on Captioning In Silverlight on the Accessible Media Project and also be sure to check out these two open source projects: