DS, are these nylon web, leather or simulated leather? Does the smell seem to be an accumulation of normal foot funk or actually something from an external source, like you stepped in something rotten? Here are some things I would try.

If the sandals are impervious to water, like all nylon straps and rubber sole, try soaking them it a strong vinegar/water solution. Vinegar does a pretty good job of neutralizing most odors. Then you just smell like vinegar.

If they are leather or otherwise not soakable, get a couple of boxes of baking soda, like Arm and Hammer. Sprinkle it on really thick and let them sit in a location where they can air - in the garage, on the porch, etc.. You might have to do this several times, turning them an sprinkling the baking soda on different parts inside and out until you have treated all the inside surfaces.

As a last resort, there are deodorizers you can get at auto parts places. Just test them in inconspicuous places to make sure they are not going to hurt anything.