Go to /Applications/Utilities/Audio Midi.app and see if you can change the bit depth/sample rate for the USB port.
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Are you using iTunes as your playback software? If so it will not automatically switch sample rates from 16/44.1 to 24/192 and as previously suggested you will have to open Audio Midi under Utilities to playback a 24/192 file and change it again for your other files.
Alternatively you can use a secondary playback software like Pure Music, Audivrana or Amarra which can work in conjunction with iTunes but will automatically switch rates. Each offers a free trial.