This script formats audio files to the Bastl Instrument's GrandPA's format, to use just install ffmpeg (brew install ffmpeg), copy it to a directory with files that end in ('.mp3', '.wav', '.aiff', '.ogg') and run it. It overwrites everything too so be careful (try first, it works for me).

grandpa_script.rb

#!/usr/bin/env ruby

# What's this?
# ============
#
# A script that converts all kinds of files to the file format Bastl Instruments GrandPA requires.
#
# Install
# =======
#
# 1. Put all your files inside of the root dir of the SD card
# 2. Copy this file to it
# 3. Open terminal and run it

cwd = File.expand_path(File.dirname(__FILE__))
supported_extentions = ['.mp3', '.wav', '.aiff', '.ogg']

# According to the manual:
#
# The files on the SD card need to be in the root directory and have specific
# names e.g.: P0.wav -P9.wav, PA.wav -PZ.wav etc. The first letter of the name
# has to be a capital P and second # letter 0-9 or A-Z (also capital).
#
# Samples have to be 22050hz, 16bit, mono wav files. (they can also be 44.1khz, but the sample rate will not
# allow pitch up then).
numbers_files = (0..9).map { |number| "P#{number}.wav" }
letters_files = ('A'..'Z').map { |letter| "P#{letter}.wav" }
grandpa_language_file_names = numbers_files + letters_files
failed_files = [] # due to the limited amount of file names we can have

('A'..'Z').each do |letters|
  grandpa_language_file_names
end

Dir.glob("#{cwd}/*").each do |file|
  next unless File::file?(file)
  next unless supported_extentions.any? { |extension| file.end_with? extension }

  if grandpa_language_file_names.any?
    grandpa_name = grandpa_language_file_names.shift
    command_to_run = "ffmpeg -y -i #{file} -acodec pcm_s16le -ac 1 -ar 22050 #{grandpa_name}"

    puts "ORIGINAL FILE: #{file} ... GRANDPA NAME: #{grandpa_name}"

    # Run command, oh ruby joy
    puts command_to_run
    `#{command_to_run}`
  else
    failed_files << file
  end
end

if failed_files.any?
  puts "The following files did not convert due to the limited amount of files names we can have: \n"

  failed_files.each do |failed_file|
    puts failed_file
  end
end

cd /Volumes/GRANDPA to go to grandpa's dir.