Do you have a garbage disposal unit? If so the mold smell can be coming from the chamber. What I do to clean out my disposal unit is fill it with ice cubes, turn it on with out running any water let it grind the ice cubes till it looks like a snow cone constancy, then run a trickle of water. The water will cause the ice to refreeze and get ground up again. The ice crystals will scourer the chamber clean.
If you do not have a disposal unit, or it is clean. The only other thing I can think of is water is getting between the counter top and the sink lip. This is more common with under mount sinks, but can happen with drop in units as well.