If you're referring to the 'C-97_KC-97_B-377.zip' download:
Not sure if or what (adverse ?) effect reducing the .wav volumn will have as output volumn is ultimately controlled in the sound.cfg file.
The file names vary depending on which variant you are editing. For the 97E the files are 'con_(1 thru 4).wav' and 'xcon_(1 thru 4).wav'.
Most of the variants use FS2002 Connie 'sound packs'. You might want to try an FS2004/FSX sound pack. The FS2004/FSX 'Reworked R-3350......' pack from here works well-
http://simviation.com/1/browse-FS+Sounds-107-0.