There is a woman in Kiloude City. She is near the top of the city, just on the left of where the lake is.
She will ask to see a trainer with a stylish, colourful outfit - and you're gonna be that stylish, colourful trainer.
She will want to see an outfit that will get you the different sound. (When you finish changing clothes, remember that sound that you hear when you dress your trainer? She will want to hear an outfit that is worth a particular sound.)
When you fulfill her pleasure, she will give you the Sundae Dress.
Also, when you wear it the sound you hear when dressing will be a really awesome trumpet sound. ;D
Hope I helped. :)