Loading samples from a SmartMedia card has a few restrictions:

• Files need to be named in order, EG: 00.WAV 01.WAV 02.WAV 03.WAV 04.WAV (extension can be upper- or lowercase)
• 100 files max in the root directory. Dunno about using subdirectories.
• 8- or 16-bit, 32KHz. .WAV or .AIFF. If you use a rate other than 32KHz playback speed will be distorted.
• You can format the card in the ES-1. It's FAT-12.
• If you get Er.3 that means you're out of space.
• Actual sample space seems to be ~2.2MiB in my experience.

With Audacity:

• Select all audio, Tracks > Resample… select or enter '32000'.
• If you want to make it mono: Tracks > Mix > Mix Stereo Down to Mono.
• File > Export > Export as WAV and set encoding to 'Signed 16-bit PCM' or 'Unsigned 8-bit PCM'

With FFmpeg:

…

With SoX (and Bash):

for file in *.wav; do sox $file -r 32000 -b [8|16] -c [1|2] [optional output directory/]$(basename $file); done

• -r is sample rate
• -c is channels
• -b is bit depth
• 'optional output directory' has to already exist

• https://files.ryliejamesthomas.net/manuals/korg_electribe_es-1.pdf
• https://www.synthmania.com/es-1.htm
• https://github.com/chris-roerig/es1-renamer
• https://www.korg.com/us/support/download/product/1/102/
  • https://www.korg.com/us/support/download/software/1/102/3317/ - ELECTRIBE-S/ES1 to .WAV PC Utility.
• Korg Electribe S (ES-1) Firmware and Factory Sounds - Links to all resources, tools on the Vintage Synth forum.
• Faxi Nadu - Korg Electribe ES1 Sampler / Sequencer Review