The antonyms for the phrase "decreasing volume" could be "increasing volume," "expanding sound," or "enhancing loudness." These terms describe a situation where the sound is amplifying, intensifying or rising in volume, rather than gradually fading away. An increasing volume denotes an escalating level of sound, which could be positive or negative, depending on the context. For instance, increasing volume can either imply a joyful outburst of music or an unwarranted noise disturbance. In any case, the antonyms of decreasing volume provide a diverse range of expressions that enable one to communicate the exact level of sound in specific settings.