The trouble?: WMP11 says the audio codec is: "Windows Media Audio 10 Professional, 128 kbps, 44 kHz, 2 channel 24 bit 2-pass VBR". 24 bits seems like overkill for a voiceover, and Silverlight 3.0 won't play it. (Or is it a container problem?)
Suggestion: In future, encode audio for WMA 9 and/or 16 bit for maximum backward compatibility.