You won't find any on Audible.com that are both WhisperSync and Spanish (I just looked), but you can search for WhisperSync books, or for Spanish books by using Advanced Search.
For WhisperSync -- Click Advanced Search (just under the main Search box on the upper right), scroll to the bottom, and select WhisperSync for Voice. Click Submit. With no other restrictions, that returns >190,000 results!
For Spanish language -- Click Advanced Search (just under the main Search box on the upper right), scroll down to Language, click Show All, and click Submit. Just under 7,000 results.
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