Noisy interior? Welcome to Miata-dom. This won't make it silent, but it's a huge step in the right direction. This is a custom, pre-cut (aside from the tape, which is easy to cut to length) kit from our friends at DEI that provides the right amount of Boom Mat. Boom Mat is a premium quality sound damping material that keeps interior panels from vibrating and generating noise by trapping that noise between an asphalt-free butyl lower layer and a 40 mil (.004" / .1mm) thick aluminum top skin (.080" / 2mm thick in total). Since Boom Mat doesn't need to cover the entire area to do its job, you spend less money and add less weight (6.0 lbs in total) to get the job done. This kit addresses the floor, rear bulkhead, and the various pieces on the package shelf. It does not include pieces for the doors, although that kit is available below. Includes instructions to show exactly where to apply it, and even includes a roller to be sure it's properly formed and adhered.
Have an NC? This will fit, although the instructions are for an NA/NB. As long as you can get your interior apart and do some problem-solving, these should work fine for your NC. Remember that the bigger and flatter the metal surface, the more important it is to use this material to damp vibrations. Also bear in mind that you don't have to cover the entire surface to get the benefit. Have an ND? The same (as the NC) applies, but the ND has quite a bit of this general style of sound deadening to start off with. While this could improve things, it would likely be by a very small degree (if at all) and isn't worth the cost or complication.
How much quieter is it? That's a surprisingly tough thing to quantify. While the decibels do decrease, it's not by much, but it's in the right range - the biggest difference is that the harshness and tone of the noise is much decreased / improved. These keep the metal from acting as transducer, which makes the car much more livable and improves the sound of the stereo - especially with the door panel kit. That's a tough thing to attach specific numbers to, but it does make a big difference to how pleasant the car is.