NMM RATCF Controller described operational deviation event when incorrect readback was missed; resulting in an altitude descent without required coordination.

Narrative

While working NMM3 in Sup 1 configuration; I descended Aircraft X to 2000 FT; then keyed up the coordination line to coordinate the inbound for Aircraft X to Key Tower; during which time Aircraft X was reading back the descent clearance. I copied the readback from Aircraft X as well as some other transmission from another aircraft. As soon as I completed the coordination for Aircraft X Heavy; I transmitted on all frequencies 'Meridian Approach off line say again.' No one replied to my transmission. ZTL then called a short time later and asked if I had descended Aircraft Y Heavy; looked at that aircraft's data block; and observed them descending out of 8000 FT. I asked Aircraft Y Heavy what altitude Center had cleared them to and they replied that I had descended him to 2000 FT. I told him 'Negative; that was for another aircraft; maintain 7;000 FT.' He read back 'Roger; maintain 7000 FT.' I then called ZTL back and informed them that Aircraft Y had taken another aircraft's descent. I then notified the Supervisor of the possible deviation.