No they're not.
I think the BBC are missing out on a great opportunity to cash in by not making them available.
I've found them all to be very good. Radio being an excellent medium for Fantasy.
I've downloaded a program called "Radio Downloader" that can be programmed to save such items. Once you mark a particular show you are interested in, it will automatically save them for you.
As well as several adaptations of Discworld books, there was also a very good one of Nation.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal