The sale volume variance looks at the effect of the actual sales being different from the budgeted sale. If the business is using absorption costing then it is costed at the standard profit per unit and is the sales volume profit variance. If they are using marginal costing the it is costed at the standard contribution per unit and is the sale volume contribution variance.
Do watch the lectures. They are a complete free course for Paper MA and cover everything needed to be able to pass the exam well.