If they're leaning back, you may be too loud. It is also useful to vary your volume during a speech, using either an increase or decrease in volume as a means of emphasis. Pitch refers to the degree our voices are high or low. Each of us has a habitual pitch, or level at which we speak most frequently, and an optimum pitch, or the level at which we can produce our strongest voice with minimal effort and that allows variation up and down the scale. Most people's optimum pitch is 4 or 5 notes up from the lowest note they can sing.
Effective speakers use their optimum pitch as a base but vary their pitch for emphasis.
Rhythm is extremely important to your delivery. It involves the characteristic pattern of your volume and pitch. This means that the speaker's voice is consistently going up and then down in pitch in a predictable an sometimes irritating pattern. Although variation in the rhythm of your voice is certainly to be encouraged, the exact pattern of variations shouldn't be predictable. This is the rate at which you produce sounds.