I didn't do it all - but I've had to modify several components.
Look at
http://www.melissi.co.uk and you'll see that it's a big system, with networking, audio, video, Unicode text, forums, and a lot more. And it has to work on big campus systems, where the security lock-downs can be extreme.
Remember, it's 12 years' worth of code. Most of it works!
The key to big programs is modularisation. Perhaps any program. This is not one great splurge of source - it's lots of modules with specific jobs. In fact, some functions are now spawned out to sub-modules. All networked speech (VoIP) is handed to a separate executable called MIPS (Melissi network speech) which can handle up to seven real-time audio streams and, at the end of a session, mix them down to one WAV or MP3 file for review. We talk to that over a localhost network connection and send a simple command set.
Going to bed now with my Kindle Paperwhite. Best bit of book-reading kit ever!
-- Jim DO IT FASTER, EASIER AND BETTER WITH AppGameKit FOR PASCAL