Cute answer, 1313...
The "missing" tune is the 14th track in the loop, "Give A Little Whistle".
The actual running order of the loop goes like this:
01 - Mickey Mouse March
02 - Minnie's Yoo Hoo
03 - Never Smile At A Crocodile
04 - Trust In Me
05 - Ugly Bug Ball
06 - The Walrus And The Carpenter
07 - See An Elephant Fly
08 - Who's Afraid Of The Big Bad Wolf
09 - The Wonderful Thing About Tiggers
10 - Winnie The Pooh
11 - Pink Elephants On Parade
12 - Bare Necessities
13 - I Just Can't Wait To Be King
14 - Give A Little Whistle*
15 - Feed The Birds
16 - Heffalumps And Woozles
17 - I Wanna Be Like You
18 - Little Black Rain Cloud
19 - Reluctant Dragon
*was never offered for sale via WDW Forever