No offense HorvatM, but that jump code is pretty simplistic. This is not a commentary on your programming skills or anything else; I've seen something like it in far too many games/demos. Everyone needs to learn somewhere, right? Also, like your post, this is just my suggestion and can be taken or left.
The problem is that gravity, and the fight against gravity that jumping entails, are both
accelerations rather than velocities. Acceleration, as I'm sure you know, is a change in velocity over time. Gravity is a constant force attempting to change the velocity of all objects. If it is resting on the ground, an object's velocity remains zero.
Rather than messing about with a flag that determines the direction of an object's velocity vector, one can simply use the sign of the number holding the velocity itself (since we are only interested in the Y component of said vector). Since coordinates on the VB's screen(s) increase away from the upper-left corner, -Y velocities/accelerations should probably mean "up," to make the code easier to grasp.
Another signed number can hold the gravitational constant. This will be added to the velocity value each frame. The velocity will in turn be added to the object's Y position, which will be constrained by the floor.
To jump, simply apply another acceleration force in a direction opposite to gravity which only persists while: A) the button is held down, and B) some amount of time has not gone by. This time value in B depends on how high you want your object (e.g. fighting bug) to jump. This and the acceleration force will have to be selected by experimentation and will likely vary by the fighter.
I'm sure HorvatM's code can easily be altered according to the above. This system, while a little more math-intensive, will give a much more realistic motion to the fighters. It should, of course, also be applied to the victims of upper-cuts and the like

Hope that helps
PS: Thanks for uploading those sounds, HorvatM. If you (or anyone else) want a whole lot more sounds that are free to use, I wholeheartedly recommend
http://www.freesound.org/. You can, of course, find a user named RunnerPack on there