Quicktime: Export .flv to .aiff trouble

I’m writing a script to batch convert the audio track of .flv files to .aiff files.

When I do this manually (open the .flv in QT and export Audio as AIFF with most recent settings) everything is fine. When I script the exact same thing, I only get the first 8 seconds of the audio in the .aiff file. My script is below. Has anyone else encountered this in QT?

tell application "Finder" to set the source_folder to (folder of the front window) as alias
set the item_list to list folder source_folder without invisibles
set source_folder to source_folder as string
repeat with j from 1 to the count of items in item_list
	set this_item to item j of item_list
	set the_path to (source_folder & this_item) as text
	tell application "QuickTime Player 7"
		set my_movie to open file the_path
		tell document 1
			export as AIFF using most recent settings to ("Macintosh HD:Users:myname:Desktop:Other:" & (this_item as text))
		end tell
	end tell
end repeat