"Offline" means the program knows about a piece of media and where it would be positioned in the project, but isn't connected to it and so it cannot be used until the connection is made.

If you move a video file to another location on your disk, outside the program that hosts the project with a reference to it, the program will indicate the events referencing the video file as "offline".
In this here case the program cannot provide the project with direct access to the audio data, since Vegas is doomed to use a proxy file for that purpose. So it just shows the empty event and flags it "offline".

I'll check back here tomorrow if any more questions have come up. Cheers and good night,

i/o