Any idea to make this thing works?

I want to make the background music fade out when it reach a specific time line. herewith my example

|normal volume |fade out 0 | normal volume|
|frame 1-100 |frame 101-150 |frame 151-200|

when i hit a button in time line 101-150 (go to) frame 1-100 or frame 151-200, it goes back to normal volume.