very fun! a couple sound design suggestions:
- try randomising the volume and pitch of the fire sound a little each time it triggers, to vary it up a bit
- the fire sound ends quite abruptly at the end of the clip, so make sure it fades a bit nicer and for longer
- movement and collision sounds for the cannon, cannonballs and boxes are a great way to give a sense of real weight and material to the world
- for collisions, you can easily do it yourself and it'll be better than freesound - record some things of different materials falling off a table onto the floor. use audacity (free audio software) to change the speed and/or pitch of the sound until it feels right for the boxes.
but it's got the makings of a good casual game, as others have said!