iBooks cannot export

Hi Guys.

The iBooks Author cannot export my book because we have non-optimized media. My question is: Where can I Know where is the media non-optimized!?

See printscreen: http://cl.ly/image/3e1N2s1o262i/o

It's in Portuguese and says something like: Cannot export for .iBook format because we have 2 files non-optimized, please correct this and try again.

Thanks,

Lucas