Halve the input audio volume:

volume=volume=0.5
volume=volume=1/2
volume=volume=-6.0206dB

In all the above example the named key for 'volume' can be omitted, for example like in:

volume=0.5

Increase input audio power by 6 decibels using fixed-point precision:

volume=volume=6dB:precision=fixed

Fade volume after time 10 with an annihilation period of 5 seconds:

volume='if(lt(t,10),1,max(1-(t-10)/5,0))':eval=frame