。
All things are perfectly still.
Literal
All-things [subject-が] have-quieted-down.
万物 is a literary compound 'ten-thousand things' = 'all things in creation' — classical/philosophical register. 静まり返る ('to fall completely silent') is a compound verb 静まる ('quiet down') + 返る ('return back,' used here as an intensifier suffix) meaning 'to go utterly quiet.' The ~ている marks the resulting state of total stillness.