You refer to "the" sandals -- is it just one pair that you're wearing over and over? Be sure to take a break and change to different shoes each day. Buy other sandals if you really want to wear sandals daily--don't keep wearing the same pair. If you try the things suggested like baking soda and rolled-up newspapers, both of which are helpful, you may need to get rid of this pair if the smell doesn't come out after you try those things.
Be sure to get sandals made of natural materials (leather, canvas, etc.) and not plastic or fake versions of leather. You'll have to pay more for natural materials but they're worth it. If you try things like a spray-on deodorant for your feet etc. and still have odor, see a doctor to see if you need treatment for excessive sweating.