The LCD is probabely multiplexed, and driving an lcd requires special signals (not just logic levels). wich will make it extremely hard, maybe even impossible, to get the data out of the lcd signal lines...
Is this a scientific calculator? or a normal one (with only +, - , * and / )?
In the latter case it would be simple to program an actual calculator into the 'F877 and just use the keypad of the one you have now, and 7-segments as output, togheter with your soundchip...