FL Studio Tutorial - How to use volume automation in FL Studio - Lesson 20
As explained in the above video, click on the left-top corner of the audio clip in your playlist. Then choose the appropriate link for volume automation.
Here do the Right-Click at the appropriate places to add 'Audio Points'. This will create a section on that. Then drag that section up or down to adjust the volume level of that particular portion.
In the above video, the volume level was brought all the way down, there by effectively muting that portion.