In addition to Tom's good advice, another method is to access the audio file in iMovie's package contents folder.
In Finder, in the Applications folder, right-click (or control-click) on iMovie.app and select Show Package Contents from the pop-up menu. Open the folder Contents>Resources. In that folder you will find components for each Trailer, including the soundtrack. The file you are seeking is named "SoundTrack - Romance.m4a".
Copy then paste the file to your Desktop (don't otherwise mess with items in package contents folders!). Open the file in QuickTime - it is titled "Love Story FC2". Opening the Inspector in QuickTime (Window>Show Movie Inspector) reveals it as AAC format. If you wish, as Tom indicated, you can Export the file from QuickTime in other sound formats, such as AIFF. If preferred, you can import either the AAC file or the converted file to iTunes (or other audio apps) for use as required.
John