This is the weirdest darn thing.
I'd recently upgraded to .153 then a little later I'd been juggling with my Now Playing list, moving stuff around suddenly MC9 couldn't find the tracks anymore. They were there, I checked with MC9's My Computer and then Explorer. I tried a diff. artist/directory and had the same problem. I restarted MC9 and the problem's gone away.
Media Center Registered 9.0.153 -- C:\Program Files\J River\Media Center\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2399 MHz MMX / Memory: Total - 523 MB, Free - 188 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1106 (xpsp1.020828-1920)
Shell32.dll: 6.00.2800.1145 (xpsp2.021108-1929)wnaspi32.dll: N/A
Ripping / Drive E: Copy mode:ModeSecure CD Type:Auto Read speed:Max
Drive F: Copy mode:ModeSecure CD Type:Auto Read speed:Max
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: No
Burning / Drive F: AOPEN CD-RW CRW4048 Addr: 1:1:0 Speed:40 MaxSpeed:40 Use MJ Engine:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: No
Use playback settings: No / Normalization: None