。
I heard a tree fall with a thud.
Literal
Tree [subject-が] thud-[adverbial-と] sound [object-を] make-[and-て] fall [nominalizer-の] [subject-が] was-heard.
どすんと is a heavy-impact mimetic — the sound of something dropping with weight. 音を立てる ('to make a sound') is the standard collocation. The whole clause 'a tree fell making a thud' is nominalized by の and becomes the subject of 聞こえる ('can be heard'), with が. A textbook example of perception verb + のが structure.