Hi. I am a bit of a NavyField modding expert.
.CXF files are really simple, you take an MP3 and change the file extension to .CXF. Easy. But the "SOUND" folder tricks you; background music is all that it contains.
Gun sounds are a bit more complex than the music sounds. Gun sounds are imbedded in .SPR files. The tool, SPRing, is capable of extracting and replacing these sounds (although it is not perfect, it gets the job done.) just like it can the graphics of NavyField. There are two SPR files that hold gun sounds, one is the file the game reads and the other one is just plain there. The active one is EFWPFT.spr; some sounds you will recognize in there will be fire_sma.wav (little DD and AA guns), 105MMCN1.wav (Mid-size CA and BB guns), 80MORT2..wav (Large AA bursts and big BB guns), and more. Another file which may interest you is EFBT.spr, which contains a large assortment of artillery fire and shell explosion noises as well as impacts and crashes. Other sound sprites are for the voiceovers, the aircraft, interface sounds, etc. Everything in NavyField that you hear is stored in a SPR file except the music.
The current version of SPRing is about 6.1; I have been updating it. While the core engine has not changed I modified the SPRInfo.txt so that SPRing can automatically find and read the new sprites (Does not work for everyone.) I established a newer, more active site modeled after the Navyfield Graphics Project at http://w11.zetaboards.com/NFSP/index/ and there I am available through the forums. You can download SPRing 5.1 here.A text file which can update the program's browser (though you still need to manually navigate to your SPR folder at least once for it to work, or ensure NF is installed in its default location) can be downloaded here.