The Sound Crossfade transition works by animating the Volume of two overlapping sound strips to evenly fade between them. Because this simply animates a value it does not create a strip like other effects or transitions.
The Cross transition fades from one strip to another, also known as a crossfade. Strips can be overlapping or have a gap between them, however, when strips contain a gap the last and first frame of each strip is extend which can cause a pause if any of the strips are a sequence.
The Gamma Cross transition is similar to the Cross transition, however, the Gamma Cross transition uses color correction while transitioning between the two strips, resulting in a smoother transition that is easier on the eyes.