Some Windows media audio streams work, while others don't (in the 10.0.84 beta build). The difference seems to be that for some reason MC is seeing some WM audio streams as a "file type" of video (or so says the "file type" field). And of course this field is uneditable so it is not possible as far as I can tell to force MC to use WMA rather than WMV.
Media Center Registered 10.0.84 -- C:\Program Files\J River\Media Center\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
AMD Athlon 1988 MHz MMX / Memory: Total - 1048 MB, Free - 503 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1400 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: 4.60 (1021) , ASPI for Win32 (95/NT) DLL, Copyright © 1989-1999 Adaptec, Inc. / Aspi32.sys: 4.60 (1021)
Ripping / Drive E: ATAPI CD-RW 52X24 Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive E: ATAPI CD-RW 52X24 Addr: 1:0:0 Speed:48 MaxSpeed:52 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: Yes
Use playback settings: No / Normalization: None