I don't have that particular aircraft, so I don't know what gun it's set up with. What gun is it referencing for the cannon?
Find that xdp file in the Guns folder and then edit the SoundName reference to whatever SoundName="c##" you like in the sounds.xml.
Perhaps,
c37_land = ai_37guna.wav, or c127_land = ai_127guna.wav.
Or create your own C## entry in the sounds.xml for these sound files that don't appear to be referenced anywhere.
ai_37gunb.wav
ai_40gun_oneshot.wav
ai_127gunb.wav
Or, you could try using,
Gun
AvHistory_gr_gun_Bk3_7-flak18 that references sound c75 for a bigger cannon sound
Weapon
AvHistory_gr_round_Bk3_7-flak18
The sound.xml has
<c75>
<no_damage>
<sound file="xgun_75mm.wav" fileInt="gun_75mm.wav" Looping="No"/>
</no_damage>
</c75>