

Look for information about 567 tone decoder chip. It is an easy to use
chip for detecting sound of certain frequency. You need only one
567 and some other componets for each led you want to control.
The chip can take the voltage levels, which the casette gives.
Radio Shack Archer Semiconductor Reference Guide gives good information
how to use that chip. It should be simplest way to go. The chip costs
something like 1-2 $.