A convex lens will take light entering it and bend it toward the principal axis. Thus the name "converging" lens.
A concave lens will take light entering it and bend it away from the principal axis. Thus the name "diverging" lens.
Remember, the principal axis is an imaginary line passing through the center of the lens.
If you place an imaginary line through the center of the balloon this would be your "principal axis".
Next imagine the sound wave's wavefront as a sheet of paper approaching this balloon along the principal axis. The sheet of paper (wave front) is perpendicluar to the principal axis but sliding along the axis toward the balloon. It will encounter the balloon first where the principal axis enter the balloon.
If the balloon is filled with carbon dioxide the wave front that enters the balloon will slow down since sound waves travel slower in denser gases. This will allow the parts of the wave front still outside the balloon to get ahead of those that are inside so that once they do enter the balloon they will be further ahead of the wave front that entered earlier. This will make the wave front go from a flat sheet of paper to one that is more cup shaped bringing more of the wave front closer (converge) to the principal axis than they would be otherwise.
If the balloon is filled with hydrogen, a less dense gas. the part entering the balloon will travel faster and form like a bubble on the sheet of paper that is trying to get ahead of the original wave front and expand away (diverge) from the principal axis.