Well I took a look and found that whoever made the changes to Cannons_init.c for that version of GOF, changed the cannon sounds, but did not create any sound alias for them. I have never played 2 or 2.5, so I was not aware of this problem. I'm generally opposed to fixing GOF 2 and 2.5, because of things like this; they either didn't finish their mods, or were incompetent. I offer them as downloads, because for some reason some people want to play those two version, but both 2 and 2.5 are not actually improvements over GOF 1.2, but rather, they took a bunch of things out, stripped it down and frankly, I think messed it up.
Anyway, it appears they wanted different sounds for cannons (e.g. cannon_fire_8), but did not create sound aliases or actually add the sound files for them. I'll put this on a list and maybe substitute what we did for ERAS 2, but it is not a priority for me, because I don't really want to support 2 or 2.5 because of their screwups. I suggest either GOF 1.2 or ERAS 2 because they are more complete and I will support those two versions. But since it appears no cannons for GOF 2.5 will sound because of this, I will probably get to substituting the ERAS sounds in at some point, because it is not a difficult fix, but I can't guarantee a timeframe at the moment.