Antonyms for the word "more thundering" would indicate a reduction in volume or intensity, such as "quieter," "subdued," or "muted." Alternatively, antonyms that suggest a change in the quality of sound could include "soothing," "gentle," or "melodic." Other possible antonyms might be those that describe a shift in emotional tone or atmosphere, such as "peaceful," "calm," or "serene." Ultimately, the antonyms for "more thundering" would depend on the context in which the word was being used and the effect the speaker or writer was trying to achieve.