It wasn't the sound of a broken window but the blinds hitting the window frame everytime someone opened or closed the door to the office. What Columbo had figured out was that 1/ What that crashing sound was on the tape recording. 2/ That the door must have been left opened (the sound heard was the murderer closing the door) and the key found on the floor ( which didn't fit any of the locks in the office )was not used to get in the office. If you remember the writer goes back to work late after the party to finish something for what ever reason the A/C isn't working so he opens the window and props the front door open so he can get a little cross ventalation going. While Columbo is investigating he is one of the last people in the office when the next to last person leaves he closes the door behind him at that moment while Columbo is sitting at the dead man's desk the blinds crash into the window frame recreating the sound which Columbo had heard earlier while playing the tape recoder.
-hbe

Thank you, Hard boiled Egg! Now it finally makes sense...