Lilie's idea is great! I wouldn't have thought of doing that, but I bet if they learn to associate that sound with a reward it will work. Dogs are great at making associations, even when we don't want to.
I get up before my husband on work days, and they stay quietly in their crates until after I take a shower and dry my hair. As soon as I shut off the blow dryer they start to whine because they know I'm going to let them out to potty, and then they get breakfast. Obviously they've made an association between the sound of the blow dryer stopping, and FOOD!