The audio dramas are a licensed continuation of the series but they are not officially "in continuity." Anything that happened or was mentioned in the 1966-1971 series is really the official DS canon. However, it's up to each individual fan to cobble together their own version of what's in "canon." These audio dramas, IMO, come closest to the "feel" of classic DS. The first one, in particular (IMO), really captures the feel of DS.