The NTW sound add-on is 67 MB large. It contains 29 sound files.
It needs to be installed manually.
Install Instructions for NTW sound add-on:
1.The first thing you need to do is convert all the .mp3 to .wav files,i have added a freeware program that you can use:dBpowerAMP Music Converter,but feel free to use any other program.

2.Then you will place them in the music file which is located here by default C:\Program Files\Total War\Medieval - Total War\Sounds.If your NTW install is located somewhere else put the files in the appropriate \Sounds file instead!


The Samurai Warlords Add-on is 110 MB large and contains about 160 sound files. It installs automatically.
That's the difference.
You've to take into account that a lot of people can't handle .rar or .zip archives, let alone know how to use a third party program to convert mp3 to wav.
It's just asked too much.