Quick suggestions: search for information on "radio dramas" rather than "audio dramas"; almost all the info that was/is relevant for radio drama production will apply.
Also search for information on "foley", which is the term for overdubbed sounds like footsteps or doors closing or whatever. Much of the info will relate to film, but it will be relevant to what you're trying to do, too.
As for having conversations sound like they are taking place in specific environments, the two basic approaches are (1) to record in a similar environment, either with an additional room mic to capture the ambience, or to have a bit of distance between performer & mic to let some of the room sound in, or (2) to record as dryly and cleanly as possible and then use reverbs, delays, etc. to alter the sound to suit.
For environmental effects, you could always use a portable recorder and record your own. There are also lots of sound libraries available - some require you to purchase an entire set, while some allow you to just purchase the bits you want.
