In the mid 80s I used to listen to The Hitchicker's Guide To The Galaxy in my car all the time on tape. I thought they were the original radio broadcasts, but having recently acquired the first CD (as the tapes are long gone) and was surprised to discover that Trillian and some other characters sounded different, there's new scenes, missing scenes, and some scenes are slightly different.
http://www.amazon.co...nal/dp/160283511X

Looks like they redid the series for LP and Trillian was recast due do a scheduling conflict.
http://en.wikipedia....album_adaptations

There's also 4 more radio series on CD - the last 3 of which were first broadcast in 2004! I plan to pick them up with future Amazon orders.
http://www.amazon.co...nal/dp/1602835454
http://www.amazon.co...ion/dp/1572704691
http://www.amazon.co...ase/dp/1572704888
http://www.amazon.co...ion/dp/1602833060