If you are working in Midi then you can select the tracks and highlight the sections for which you need to reduce Gain. Then use Edit > Change Velocities using a pecentage lower than 100% say 75%. Depending on the Synth you are using, this may change the character of some of the sounds.

If working in Audio, it depends on which stage you have reached in development as there are a number of options. These options are likely to be more complex than for Midi.