A quick, maybe imperfect solution – use an expression in the volume filter, e.g. for linearly decreasing volume between from 4s to 5s:
volume=1-0.85/(5-4)*(t-4):eval=frame
For t=4, the volume will evaluate to 1, for t=5 it will be 0.15:
for t=4: 1 - 0.85 / (5-4) * (4-4) = 1 - 0.85 / (10-4) * 0 = 1 - 0 = 1
for t=5: 1 - 0.85 / (5-4) * (5-4) = 1 - 0.85 = 0.15