Sounds as though something unfortunate has happened to your sounds.xml. Use Notepad to open xdp and xml files. If you make any changes to xdp files, always delete the corresponding bdp file.
Open the xdp file of the aircraft you get this with. Look and see which guns it uses, and then locate the corresponding .xdp in your 'Guns' folder.
You should see this entry, for a single British .303 Browning:
SoundName="c303"
Then open your sounds.xml, and check for these lines:
<c303>
<no_damage>
<sound file="xgun_303_browning.wav" FileInt="gun_303_browning.wav" Stereo="Yes"/>
</no_damage>
</c303>
Then look in your sounds folder for the corresponding sound files. Check they are actually what it says they are.
And then you'll know what is not working correctly.