I occasionally add cardboard to my compost but I've never noticed a soapy smell. But I don't soak it either. I wouldn't worry about it unless you have other reasons to think it might be contaminated.
More information: Don't use cardboard that has a lot of printing, color glossy printing, a plasticky coating or that appears to be contaminated. Greasy cardboard board shouldn't be used. Be sure to remove any parts that are not cardboard (labels, windows, etc) and peel off any tape.
