ZAU Controller described similar call sign event that resulted in an uncoordinated descent into an adjacent sector's airspace.

Narrative

I had both Air Carrier XXXX and Air Carrier XXYX on frequency inbound to ORD. I was unaware of similar call signs at the time I issued a descent clearance to Air Carrier XXXX to FL250. After the clearance was issued and read back by Air Carrier XXYX; Air Carrier XXXX questioned the clearance and advised I had similar call signs on frequency. I tried to get Air Carrier XXYX to maintain altitude but he had already started his descent in Cleveland Centers airspace. I called the ZOB sector to advise what happened and then got the aircraft on frequency to the appropriate altitudes. Recommendation; similar call signs are always a risk. I am not sure why I didn't realize they were both on frequency but I didn't. Even when questioned by the correct pilot; and I realized the mix up; I still didn't hear clearly the correct call sign. Our frequencies are not always crystal clear and the voice inflections of pilots and controllers can easily make Air Carrier XXXX sound like Air Carrier XXYX. We will always have similar call signs in this business; I guess the best recommendation would be to be as alert to this possibility as one could be so it doesn't bite you.