What type of underlayment should be used when installing engineered floors?
There are several good foam type underlayments available. For maximum sound proofing, I'd probably recommend one of the high density closed cell foam pads as opposed to the standard blue or white 1/8" lightweight styles.  You may have to visit a flooring specialty store, I have never seen the better grades in stock at the box stores, although they may be able to order it for you.  The product description on the better grades will specifically mention superior sound deadening. Good luck.