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A quick, maybe imperfect solution – use an expression in the volume filter, e.g. for linearly decreasing volume from 1 to 0.15 in the time interval between 4^th and 5^th second:

volume=1-0.85/(5-4)*(t-4):eval=frame

For time 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 

You may use (nested) if functions for evaluating the volume value in different time intervals of your audio stream.

For example, to keep the value 1 for the first 4 seconds you may extend the previous formula

              1-0.85/(5-4)*(t-4)

with the help of the lt (less than) function into

if(lt(t\,4),1,1-0.85/(5-4)*(t-4))

and then – for keeping the value 0.15 after 5^th second – into

if(lt(t\,4),1,if(lt(t\,5),1-0.85/(5-4)*(t-4),0.15))

And so on, always rewriting the last item with a new if function.

Of course, the resulting expression is not the optimal one, but the procedure is pretty straightforward.