The sound schemes, if they are going to be representative, should be worked out in advance. Placing the units in the correct slot so that the right sounds are connected to the right units is not difficult, but is hell if you haven't planned for it from the start.
Masis is correct about the techs. Draw some connections on paper first. Then see how they work in actual play. Revision is inevitable. Some planning in regards to the unique effect of certain techs is warranted. You can rename Railroad to stoneworking if you want to, but if a civ has it, its settler units will still build railroads. I will never forget a bad experience with an otherwise-good looking scenario set in the Age of Imperialism. The map, units, tech; all were pretty well done and I was looking forward to playing the scenario. A Russian city was taken by Germans and boom! out pops a bunch of ZULU units! The scenario's author had placed the Zulu units in the Partisan slot, and Zulu hordes in the Ukraine was the result.
I never played that one again. <IMG SRC="http://forums.civfanatics.com/ubb/hmph.gif" border=0>
One principle to keep in mind is the idea of limited means. There are only so many different terrain types, so planning which types will exist ahead of time is good. There are only so many unit-slots, so "special" unit types that you might well cherish, sometimes must be abandoned. There are only so many tech slots, and some should be used very carefully, so tech tree planning, while not absolutely essential, is certainly advisable.
Scenario design generally, and depending on your innate proclivities, is both an ordered and a chaotic process. Written-out planning is good, but it is only the first step; be ready to improvise like a son-of-a-***** when necessary. <IMG SRC="http://forums.civfanatics.com/ubb/wink.gif" border=0>
Hope this helps,
Exile
[This message has been edited by Exile (edited June 26, 2001).]