Audio problem: When Fat Bastard bursts through Austin's apartment door, he pulls a revolver out of a box. When he does, the sound of the hammer cocking is heard, but he never cocks the hammer. It stays down for the entire scene.
Revealing mistake: When Fat Bastard smashes through the door of Austin's apartment (in 1999), the door doesn't have any holes drilled in the side for the latch. You can see this when a piece with the door knob falls to the floor. In addition, you can see on the right side of the door, a mark in the slab where it was cut for Fat Bastard (Mike Myers) to slam through it.
Answer: The music is the theme song to the TV show "Charlie's Angels".