now there are two solutions

a – Add a video track  to flv file and then process with flvtool2  it will add keyframes
b – Patch flvtool2 to support audio only keyframes

we will talk here about option # b

1. Install ruby from here :  http://rubyforge.org/projects/rubyinstaller/ – get oneclick installer

2. now download ruby source http://svn.inlet-media.de/svn/flvtool2 , use svn tool like [ http://tortoisesvn.net/downloads ]

3. browse to flvtool2 source directory and then to trunk folder and double click on setup.rb

4. next apply the patch to files use the svn client to apply the patch / diff
   

   Index: trunk/lib/flv/stream.rb
   ===================================================================
   — trunk/lib/flv/stream.rb    (revision 67)
   +++ trunk/lib/flv/stream.rb    (working copy)
   @@ -43,6 +43,7 @@
   :type_flags_audio,
   :type_flags_video,
   :tags,
   +                  :audio_keyframe_mode,
   :stream_log

   def initialize(in_stream, out_stream = nil, stream_log = false)
   @@ -69,6 +70,7 @@
   @extra_data = ”
   @tags = []
   end
   +      @audio_keyframe_mode = false
   end

   @@ -203,9 +205,20 @@
   end

   def keyframe_video_tags
   +      @keyframe_video_tags_cache ||= keyframe_audio_tags if @audio_keyframe_mode
   @keyframe_video_tags_cache ||= @tags.find_all do |tag|
   tag.kind_of?(FLVVideoTag) && tag.frame_type == FLVVideoTag::KEYFRAME
   end
   +       end
   + def keyframe_audio_tags
   +        result = nil
   +        if !has_video? && has_audio?
   +           result = []
   +           src = audio_tags
   +           # Get each 28 audio tag (~0.5 sec)
   +           (0 .. src.size – 1).step(28) {|i| result << src[i] }
   +       end
   +      result
   end

   def audio_tags
   @@ -286,7 +299,8 @@
   end

   def lastkeyframetimestamp
   –      return nil unless has_video?
   +      return nil if keyframe_video_tags.nil?
   +      #return nil unless has_video?
   (keyframe_video_tags.last.nil? || keyframe_video_tags.last.timestamp.nil?) ? 0 : keyframe_video_tags.last.timestamp / 1000.0
   end

   Index: trunk/lib/flvtool2.rb
   ===================================================================
   — trunk/lib/flvtool2.rb    (revision 67)
   +++ trunk/lib/flvtool2.rb    (working copy)
   @@ -29,7 +29,8 @@
   module FLVTool2

   SHELL_COMMAND_NAME = (RUBY_PLATFORM =~ /win32/) ? ‘flvtool2.exe’ : ‘flvtool2’
   –  SWITCHES = %w{ s v r p x c i o k t n l a }
   +  SWITCHES = %w{ s v r p x c i o k t n l a y }
   +  #SWITCHES = %w{ s v r p x c i o k t n l a }
   COMMANDS = %w{ U C P D A V }

   def self.parse_arguments
   @@ -50,6 +51,7 @@
   options[:in_point] = nil
   options[:out_point] = nil
   options[:keyframe_mode] = false
   +    options[:audio_keyframe_mode] = false
   options[:tag_file] = nil
   options[:tag_number] = nil
   options[:stream_log] = false
   @@ -84,6 +86,8 @@
   options[:out_point] = ARGV.shift.to_i
   when ‘k’
   options[:keyframe_mode] = true
   +          when ‘y’
   +            options[:audio_keyframe_mode] = true
   when ‘t’
   options[:tag_file] = ARGV.shift
   when ‘n’
   @@ -214,6 +218,7 @@
   puts ”  -t path       Tagfile (MetaTags written in XML)\n”
   puts ”  -v            Verbose mode\n”
   puts ”  -x            XML mode instead of YAML mode\n”
   +    puts ”  -y            Try to generate keyframes for audio only files automaticaly\n”
   puts “\n”
   puts “REPORT BUGS at http://projects.inlet-media.de/flvtool2”
   puts “Powered by Riva VX, http://rivavx.com\n”
   Index: trunk/lib/flvtool2/base.rb
   ===================================================================
   — trunk/lib/flvtool2/base.rb    (revision 67)
   +++ trunk/lib/flvtool2/base.rb    (working copy)
   @@ -42,6 +42,7 @@
   end

   process_files(options) do |stream, in_path, out_path|
   +          stream.audio_keyframe_mode = options[:audio_keyframe_mode]
   write_stream = false
   options[:commands].each do |command|
   write_stream = true if send( command, options, stream, in_path, out_path )

   save the above text as flvtool2_audio.diff in trunk folder and then apply the patch , the patch may not work if the source is updated or a new version of flvtool2 is released, in this case you will have to apply the patch manually

5. now open command prompt and type

   exerb "path\to\recipefile\flvtool2.exy"

   flvtool2.exy will be already in the trunk directory , if the above command fails it means you do not have exerb go here  [ http://exerb.sourceforge.jp/index.en.html ]
   
   

6. Now you will have flvtool2.exe now to add keyframes to audio files  use commandflvtool2 -U -y “filepath.flv”yes -y option add key frames to audio files, key frames will be added after each 0.5 sec

Download

Download Flvtool2 revision 67 source with patch applied : flvtool2 Revision 67
Patch file : flvtool2_Audio_Patch