Q: undelete audio track on quicktime 7

Open the file and then use Undo (command-z) from the Edit menu.

In QuickTime 7 in the properties window I deleted one of the audio tracks... The file size has not reduced, so I was wondering is there a way I can retrieve it or undelete it?

That depends on your workflow specifics.

For instance...

1) How are you checking/measuring "file size" differences? Here are the stats for a file I am currently working on:

Note that the Finder indicates the files size to be 463.9 MB but the "Inspector" window indicates 380.54 MB for a file containing one video track, 2 audio tracks, and one text track. However, after deleting the secondary AC3 audio track from this file and saving the remaining data to a new file container we see the following stats...

Note that the Finder now shows the file size for the edited file to be reduced to 401.2 MB but the "Inspector" window still shows the file to contain the same 380.54 MB of data even though the "Movie Properties" window clearly confirms that the secondary AC3 audio track is no longer present. The reason for this is that the Finder "Info" window indicates the "total file size" while the "Inspector" window only indicates the amount of "total enabled data" contained within the file. Therefore, if you are using the "Inspector" window as the basis for your comparison and the deleted track was never enable before deletion, then the "file size" doesn't change when deleting a disabled audio track. Thus, if your original file is still available, compare its "Finder" window file size reading with that of the new file to see the actual amount of space each file occupies on your storage device.

2) If you deleted the original, unedited file, then you cannot "recover" the deleted audio track since the deleted audio data was never written/copied to the newly created version of the file. On the other hand, of the original file is still available, then it should still contain the data missing from the edited copy of the file.