Restoring the Lost Material
What you need
1. The commercial CD release. You really need to own the commercial CD release of HHGTTG: series 1, and series 2. As far as I know, all of the CD versions are the same, so an earlier edition will be just as good.
2. A good sound editing program. I recommend you use the free program Audacity, which is a non-linear sound editing program. Other software is suitable, and if you happen to be a professional musician or editor, you will no doubt already have even better editing programs on your computer. But if you are not an expert, Audacity is your best bet.
3. A CD ripping program. You need this to copy the sound files off the commercial CDs. CDex will do this for you if you use Windows. On Linux you can use cdparanoia or grip. On MacOS, I believe that iTunes has the required functionality. It is best to "rip" each episode as a single audio file, ideally uncompressed (.WAV).
Restoring Episode 3 by hand
One scene is missing from Episode 3. To restore it, you must first copy Episode 3 from the commercial CD. (It's on CD 2 of the "Primary Phase".) Try to copy the entire episode as a single audio file.
Open this file in Audacity. This is how it should look:
Click "Selection Start" (highlighted in yellow) and select "hh:mm:ss + milliseconds". This ensures that Audacity will use minutes, seconds and milliseconds to identify positions within audio files. Also, click the circle next to "End".
Now, download the "original version". It is a file named "magrathea-original.mp3", found on the page for episode 3. Here is the relevant data.
Next, select File -> Import... -> Audio... Navigate to the location of "magrathea-original.mp3" and select it (you might need to change the file type from "WAV, AIFF..." to "All supported types").
You will now see two tracks in the editor. Select all of the shorter track ("magrathea-original") and choose Edit -> Cut. All sound data in the shorter track should disappear.
Select the first track, and then click on the Selection End box. Enter the end point: it is 16 minutes, 31 seconds, and 721 milliseconds according to the information above (16:31.721). Next, click on Selection Start box. Enter the start point: it is 16 minutes, 8 seconds and 742 milliseconds (16:08:742). Now you should see something like this:
Select Edit -> Paste. A section of the file is replaced:
That's it! The original scene has been restored to episode 3! You can now use Audacity to listen to the episode, or to export an MP3 or WAV file to keep.
Notice that you did not need the "CD version" MP3 file, or the location of the scene in the original episode (ending at 17:47.077 in this case). That information is just there so you don't need to dig out your commercial CDs in order to make a comparison.
Restoring Episode 4 and Episode 12
The procedure for restoring scenes in Episodes 4 and 12 is identical to that described above. The replacement audio clips can be found on the pages for each episode, 4 and 12, along with the start point and end point information that you need. Download the "original version" MP3 file and paste it over the "CD version" location in Audacity. If you're restoring several scenes, do the last scene first, because each replacement changes subsequent locations.
Experimental: Restoration Program
I've written a program that will do the restoration work for you. It's experimental software - you might say "alpha".
It will only run on a Linux computer, and it requires two programs to be installed: wget and mpg123. These are found on most Linux systems, and if they're not already on yours, you can install them using your package manager (e.g. APT). Sorry, there is no Windows or Mac version at present, although the software should run on both if you have the geeky know-how required to install the other programs it uses.
To use the program, extract the audio from episodes 3, 4 and 12 in WAV format. The WAV files must be CD quality, i.e. 44100Hz, stereo, 16 bit. You need to call the files 03.wav, 04.wav and 12.wav and put them in a working directory with plenty of space - a couple of Gb free at least. /tmp may be a good choice.
You can extract the episodes from the commercial CDs using "cdparanoia". For example, the following command extracts track 12 onwards to a file called 12.wav:
(That's the setting for episode 12 on the edition of the commercial CDs that I have; it occupies tracks 12 through 20.)cdparanoia -- 12- 12.wav
When you have 03.wav, 04.wav and 12.wav, download the program, and save it in the same directory as the WAV files. It is called "reprocess.py".
Run it as follows:
python reprocess.py patch
It should do everything for you automatically. It will fetch the MP3 files from this website and patch them into the WAV files as appropriate, restoring the episodes as originally broadcast. They will be called 03_original.wav, 04_original.wav, and 12_original.wav. Enjoy!