Do you want this for one sound (for your own project), or all weapon sounds (for FS)? Weapons have indexes into entries in sounds.tbl (e.g. weapon X uses sound definition Y when it fires, and sound definition Z when it hits something). Entries in sounds.tbl specify the distance sounds are audible from.
IIRC, there have been some updates to sounds.tbl, and you'd need to use a different format to modify entries than you use to define new ones. The tbl file also has comments that specify which entries go with which sounds, but some of the weapon names are outdated beta names that were changed for the FS2 release.
Also see
http://www.hard-light.net/wiki/index.php/Sounds.tbl(BTW, if odd forum bugs are cropping up- such as gradually increasing font size, or blurry icons, or slightly-rotated images- it's because one of the admins has mental issues related to April Fools' day)