Hmm, strange. If the track simply isn't found, the error you'd see at the dev console would be "Failed to load sound "*#music/track01.mp3", file probably missing from disk/repository", so if you're not seeing that error then I'm not sure what could be wrong.
My MP3 rip of track01.mp3 is 44KHz stereo 256kbps CBR, is yours any different? Early on I tested with a variety of MP3 encoding settings and Source Engine seemed to handle it fine...