the voice is not supported. the supported codecs are listed in the SDK materials under a section 'supported media formats and protocols' -- specifically it states below. hope this helps!
The following formats are not supported.
- Interlaced video content
- Windows Media Screen
- Windows Media Audio Professional
- Windows Media Voice
The MediaElement object supports the following formats. These encodings are supported regardless of the file's extension.
- WMV1: Windows Media Video 7
- WMV2: Windows Media Video 8
- WMV3: Windows Media Video 9
- WMVA: Windows Media Video Advanced Profile, non-VC-1
- WMVC1: Windows Media Video Advanced Profile, VC-1
- WMA 7: Windows Media Audio 7
- WMA 8: Windows Media Audio 8
- WMA 9: Windows Media Audio 9
- MP3: ISO/MPEG Layer-3
- Input: ISO/MPEG Layer-3 compliant data stream
- Channel configurations: mono, stereo
- Sampling frequencies: 8, 11.025, 12, 16, 22.05, 24, 32, 44.1, and 48 kHz
- Bit rates: 8-320 kbps, variable bit rate
- Limitations: "free format mode" (see ISO/IEC 11172-3, sub clause 22.214.171.124) is not supported.
i'm not sure what specific codecs are on the roadmap, but i know that the team is continually evaluating the needs of customers and balancing that with the desire to keep silverlight lean. as the progress of v1.1 progresses i'm sure we'll see more. but for v1 for sure, that is not a targeted codec support.