The requirements for playback of audio files on the S970 and S770, as shown in the Owner's manual, are:
.wav (WAV format: 44.1 kHz sample rate, 16 bit resolution, stereo)
.mp3 (MPEG-1 Audio Layer-3: 44.1/48.0 kHz sample rate, 64 to 320 kbps and variable bit rate, mono/stereo)
Note that only .wav files can be used for Audio Link Multi Pads. Additionally the playback tricks 'Time Stretch' and Pitch Shift' won't work with MP3 files sampled at 48.0 kHz.
Regarding the OP's problem, I would check the length of the filenames first, as Toril suggested.
Regards
Ian