[Moppie]

There is little you can really do to quiten down a 91 Civic.
Its a very small light weight car designed for cheap mass production.
The floor and fire wall already has quite thick carpet with sound deadening material behind and underneath it. Adding more to it won't make a lot of differnce, as most of the noise you here is coming from the doors which don't have sound protection, and from wind noise outside the windows.
You can't fit any material inside the door with out interfering with the door locks and windows, and there is nothing you can do about wind noise.

If you really like the car then try a nice stereo and use music to cover the noise, other wise if noise really bothers you (and you must be a little sensitive to it) then you would have to buy a heavier more comfort orintated car, which of course means losing all the advantages of the Civic.