I have the same CPU problem with builds 313 through 316, and it happens with streaming MP3. (Edit: I just found out that it can happen from the moment I launch MC, even before anything is playing.) It's 100% reproducible.
Media Center Registered 9.1.313 -- C:\Program Files\Media Center\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
AMD Athlon 1399 MHz MMX / Memory: Total - 1048 MB, Free - 691 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1276 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: N/A
Ripping / Drive P: Copy mode:ModeSecure CD Type:Auto Read speed:4
Drive R: Copy mode:Normal CD Type:Auto Read speed:4
Digital playback: Yes / Use YADB: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: No / Play sound after ripping: Yes Soundfile: C:\WINDOWS\Media\tada.wav
Burning / No burners found.
Test mode: No / Eject after writing: No / Direct decoding: Yes / Write CD-Text: No
Use playback settings: Yes / Normalization: None
Edit #2: I just found out that if I log in as a limited user and run MC, the CPU problem occurs. If I log in as a limited user, and run MC with Administrator credentials (using the "Run as" functionality of WinXP), the problem is absent! I have no idea why this would be.