Sorry to hear you are having an issue with your DX7 - but no one could properly diagnose your unit from just the description that you have given. There simply is not enough of a description - for all we know it could be a blown component in your speaker system (to eliminate that, listen in headphones).
On which sounds does this occur? If it is "all of them" then say so, even that would be helpful... but it does not eliminate a possible problem with your speaker system.
The only description you give is "muffled" - sounds with clarity and could be caused by an EQ set to remove frequencies between 260Hz ~ 520Hz the approximate range you describe... or a bad speaker.
There is nothing internal to the DX7, that would cause just this region of the keyboard to be muffled (that I am aware of)... you may want to visit an authorized Service Center (unfortunately it is out of warranty!)
Maybe setting of Key Level Scaling, or dirty contacts so velocity doesn't work well, or some component inside the circuit... You can control DX7 from external keyboard or from sequencer to exclude keyboard problem. You should also compare with the other DX7 loaded with the same patches on the same sound system... Check also different patch bank. If it doesn't help then there's time to open it, reseat and clean all connectors... If it doesn't help, that's a time for good technician to analyze circuits.