I think your problem is here
boolean digitalReadMux(int channel);
digitalReadMux is being used as a function later on so it needs a body. Instead of the semicolon there should be an open curly brace { some code which returns true or false depending on the mux input then a closing curly brace }
Also where you check for it you should not use ==1 as it is returning boolean rather than an integer
if (digitalReadMux(1)) { // ..... // }