。
On cloudy days, sounds carry better than on sunny days.
Literal
Cloudy-sky [possessive-の] day [topic-は] clear-weather [possessive-の] time [than-より] sound [subject-が] well heard [copula-のです].
曇天 (どんてん, 'overcast sky') is the formal weather term. 晴天 (せいてん, 'clear weather') is its counterpart. より ('than') sets up the comparison. よく聞こえる ('is heard well, carries well') uses the intransitive 聞こえる. のです adds the explanatory tone — this is sharing an interesting fact. A science-adjacent observation about acoustics.