Sound effects which are used frequently can sound painfully repetitive very quickly. This is most obvious with effects like gunshots or footsteps, which might be heard several times each second, but can be a problem with any repeated sound. Pitch shifting can be used to mitigate this.
Pitch shifting is simply the technique of dynamically shifting a sound effect up or down in pitch. Typically only a single parameter is used: a pitch multiplier. A value of 1 corresponds to the original pitch, with values below this resulting in a lower pitch and values above resulting in higher pitch. Note that doubling pitch is equivalent to increasing by an octave, so a multiplier value of 0.5 will be a decrease by an octave, and a value of 4 will be an increase of 2 octaves.
Altering the pitch of sound effects will help to make them less repetitive, but can also serve other purposes. Here are a few ways to use pitch shifting:
Random pitch shifting
Each time a particular sound effect is played, randomly shift its pitch up or down by a small amount. The actual change to the pitch may not be noticeable, but this adds a very natural kind of variation that reduces repetitiveness and can improve game feel.
When several actions are chained together such as in a combo, successively increase the pitch of the associated sound effect. This can help reinforce the length of the chain, as well as increase satisfaction. There are 12 semitones in an octave, so an increase by a single semitone corresponds to a pitch multiplier of 2^(1/12) or about 1.06 and hence you can use a multiplier of 1.06^N to shift by N semitones. A semitone makes for a natural sounding pitch step for a rising or falling sequence.
Pitch as information
A lot of 'juice' like particle effects or screenshake work because they help to convey information to the player, reinforcing the events occurring in the game, and responding to the player's input and actions. Pitch can be used as a kind of aural juice. The harder a ball hits the ground, the higher pitch the sound it makes. In much the same way, pitch shifting can be applied to collisions as well as many other types of interaction to provide the player with information about their strength or speed.
Pitch shifting is a simple but powerful technique to have in your arsenal! There are all kinds of inventive ways you can use it to make your sounds more interesting and your game feel better.