Small scripts to transcode any file VLC or ffmpeg can play to mp3.

ffmpeg-mp3-converter.rb

#!/usr/bin/env ruby -rubygems
#
# Author: Riyad Preukschas <riyad@informatik.uni-bremen.de>
# License: Mozilla Public License 2.0
#
# Transcode any file ffmpeg can play to mp3.

require 'optparse'
require 'shellwords'


#
# helpers
#

def puts_error(message)
  puts "Error: #{message}"
end

def puts_warning(message)
  puts "Warning: #{message}"
end


#
# parse options
#

options = {
  :bit_rate => 192,
}
begin
  OptionParser.new do |opts|
    opts.banner = "Usage: convert2mp3 [options] FILE [<FILE ...>]"

    opts.separator ""
    opts.separator "Conversion Options:"

    opts.on("-f", "--[no-]force", "Overwrite mp3 file if it exists already") do |f|
      options[:force_overwrite] = f
    end
    opts.on("-o", "--output-directoy [DIRECTORY]", "Save converted files to this directory") do |out_dir|
      unless File.directory?(out_dir)
        puts_error %Q(Wrong output directory. "#{out_dir}" is not a directory.)
        exit 1
      end
      options[:output_dir] = out_dir
    end

    opts.separator ""
    opts.separator "mp3 Options:"

    opts.on("-b", "--bit-rate [RATE]", Integer, "Bit rate in kbit/s (default: #{options[:bit_rate]})") do |b|
      options[:bit_rate] = b
    end

    opts.separator ""
    opts.separator "General Options:"

    opts.on_tail("-h", "--help", "Show this message") do
      puts opts
      exit 0
    end
    opts.on_tail("-v", "--[no-]verbose", "Be more verbose") do |v|
      options[:verbose] = v
    end
  end.parse!
rescue OptionParser::InvalidOption => e
  puts_error e.message
  exit 1
end


#
# gather source files
#

if ARGV.empty?
  puts_error "No files given."
  exit 1
end
source_files = []
while !ARGV.empty?
  source_path = ARGV.shift
  unless File.exists?(source_path)
    puts_warning %Q(Skipping "#{source_path}". It doesn't exist.)
    next
  end
  unless File.file?(source_path)
    puts_warning %Q(Skipping "#{source_path}". It's not a file.)
    next
  end
  source_files << source_path
end


#
# ffmpeg command line options
#
FFMPEG = 'ffmpeg'
TRANSCODING = '-vn -ab BIT_RATEk -vbr on -f mp3'.
              gsub('BIT_RATE', options[:bit_rate].to_s)
VERBOSITY = options[:verbose] ? '-v info' : '-v quiet'

if `which #{FFMPEG} > /dev/null` && $? != 0
  puts_error "Couldn't find ffmpeg."
  exit 1
end

source_files.each do |source_file|
  source_dir = File.dirname(source_file)
  output_dir = options[:output_dir] || source_dir
  output_file = File.join(output_dir, "#{File.basename(source_file, ".*")}.mp3")
  if File.exist?(output_file)
    if options[:force_overwrite]
      puts_warning %Q(Output file exists. "#{output_file}" will be overwritten.) if options[:verbose]
    else
      puts_warning %Q(Output file exists already. Skipping "#{output_file}".)
      next
    end
  end

  puts %Q(Converting "#{source_file}")

  unless system("#{FFMPEG} #{VERBOSITY} -y -i #{source_file.shellescape} #{TRANSCODING} #{output_file.shellescape}")
    puts_error "Conversion failed. Try using the -v option for more detailed output."
  end
  puts if options[:verbose]
end

vlc-mp3-converter.rb

#!/usr/bin/env ruby -rubygems
#
# Author: Riyad Preukschas <riyad@informatik.uni-bremen.de>
# License: Mozilla Public License 2.0
#
# Transcode any file VLC can play to mp3.

require 'optparse'
require 'mkmf'
require 'shellwords'


#
# helpers
#

def puts_error(message)
  puts "Error: #{message}"
end

def puts_warning(message)
  puts "Warning: #{message}"
end


#
# parse options
#

options = {
  :bit_rate => 192,
  :sampling_rate => 44100,
}
begin
  OptionParser.new do |opts|
    opts.banner = "Usage: convert2mp3 [options] FILE [<FILE ...>]"

    opts.separator ""
    opts.separator "Conversion Options:"

    opts.on("-f", "--[no-]force", "Overwrite mp3 file if it exists already") do |f|
      options[:force_overwrite] = f
    end
    opts.on("-o", "--output-directoy [DIRECTORY]", "Save converted files to this directory") do |out_dir|
      unless File.directory?(out_dir)
        puts_error %Q(Wrong output directory. "#{out_dir}" is not a directory.)
        exit 1
      end
      options[:output_dir] = out_dir
    end

    opts.separator ""
    opts.separator "mp3 Options:"

    opts.on("-b", "--bit-rate [RATE]", Integer, "Bit rate in kbit/s (default: #{options[:bit_rate]})") do |b|
      options[:bit_rate] = b
    end
    opts.on("-s", "--sampling-rate [RATE]", Integer, "Sampling rate in Hz (default: #{options[:sampling_rate]})") do |s|
      options[:sampling_rate] = s
    end

    opts.separator ""
    opts.separator "General Options:"

    opts.on_tail("-h", "--help", "Show this message") do
      puts opts
      exit 0
    end
    opts.on_tail("-v", "--[no-]verbose", "Be more verbose") do |v|
      options[:verbose] = v
    end
  end.parse!
rescue OptionParser::InvalidOption => e
  puts_error e.message
  exit 1
end


#
# gather source files
#

if ARGV.empty?
  puts_error "No files given."
  exit 1
end
source_files = []
while !ARGV.empty?
  source_path = ARGV.shift
  unless File.exists?(source_path)
    puts_warning %Q(Skipping "#{source_path}". It doesn't exist.)
    next
  end
  unless File.file?(source_path)
    puts_warning %Q(Skipping "#{source_path}". It's not a file.)
    next
  end
  source_files << source_path
end


#
# VLC command line options
#

# using find_executable() will create a mkmf.log file :(
vlc_paths = [
  'vlc',
  '/Applications/VLC.app/Contents/MacOS/VLC',
  File.expand_path('~/Applications/VLC.app/Contents/MacOS/VLC'),
]
VLC = vlc_paths.find { |path| find_executable0(path) }
INTERFACE = '-I dummy'
PLAYBACK = '--no-loop --no-repeat --play-and-exit'
TRANSCODING = '--sout="#transcode{vcodec=dummy,acodec=mp3,ab=BIT_RATE,channels=2,samplerate=SAMPLING_RATE}:standard{access=file,mux=raw,dst=\'DESTINATION\'}"'.
              gsub('BIT_RATE', options[:bit_rate].to_s).
              gsub('SAMPLING_RATE', options[:sampling_rate].to_s)
VERBOSITY = options[:verbose] ? '' : '-q 2>/dev/null 1>/dev/null'

if VLC.nil?
  puts_error "Couldn't find VLC."
  exit 1
end

source_files.each do |source_file|
  source_dir = File.dirname(source_file)
  output_dir = options[:output_dir] || source_dir
  output_file = File.join(output_dir, "#{File.basename(source_file, ".*")}.mp3")
  if File.exist?(output_file)
    if options[:force_overwrite]
      puts_warning %Q(Output file exists. "#{output_file}" will be overwritten.) if options[:verbose]
    else
      puts_warning %Q(Output file exists already. Skipping "#{output_file}".)
      next
    end
  end

  puts %Q(Converting "#{source_file}")

  unless system("#{VLC} #{PLAYBACK} #{INTERFACE} #{TRANSCODING.gsub('DESTINATION', output_file.gsub(/'/, ""))} #{source_file.shellescape} #{VERBOSITY}")
    puts_error "Conversion failed. Try using the -v option for more detailed output."
  end
  puts if options[:verbose]
end

Caveat: in its current form it only works and has been tested on OS X. 😥
Thanks to Kris Hom for the inspiration. 😄

Update:

• Check whether VLC is installed
• Should also work on Linux now
• Increase default bit rate to 192kbit/s
• Fixed bug where the file/playlist would repeat endlessly

Update:

• Also look for VLC in ~/Application/

Update:

• Added a ffmpeg version