The opposite of decreased volume could be increased volume. This suggests that the sound, size, or amount of something has become larger or more substantial. Another antonym for decreased volume is amplified volume, which connotes a similar idea of increasing the strength or intensity of the sound. Alternatively, full volume could be an antonym for decreased volume which suggests that the sound is at maximum or not reduced. The opposite of decreased volume also depend on the context. For instance, the opposite of decreased volume in finance might be escalating expenditure or growing sales. In summary, the antonyms for decreased volume can vary according to the subject matter at hand.