The shirt pocket is going to pick up...shirt pocket noise and anything rubbing the surface of the recorder. Crowd noise too, but no more or less than external omnis. Meanwhile, the built-in omni mics on the PCM-M10 do have a bass cutoff--at 80 Hz I believe--that makes things a little tinny, though tolerable if you're desperate. The little SoundPro mics above, even clipped to a shirt collar, would be superior.