Is there a calculator for converting a circuit designed for 15v to 12v?
Is there a calculator for converting a circuit designed for 15v to 12v?


No.
Alas.


  I would like to build an audio circuit designed for +/-15v, but using a +/-12v power supply. The design is discrete and all components are fine running off 12v, but will the response of the design be thrown off with no substitutions?


It may be. Or may not. There is no certain rul, unfortunately.
It is likely that a device which appears to "run fine" will be running closer to limits than before and this may only show in limiting cases - max power, higher ambient temperature. high or low mains voltage. Or distortion may be increased without it being obvious from a simple test. Or, all may be well :-).

Some of a  circuit's values  MAY be able to be "just scaled down" but there is no guarantee that this will be the case and there are many situations where it will not be the case.

In many designs, much of the circuitry is not overly sensitive to supply voltage, and in voltage critical portions there may be secondary voltage regulation achieved with eg zener diodes or regulators.

To see how a circuit will be affected by such a change really needs inspection of the circuit dfiagram and an understanding of what is being achieved at each point. For example:  


Transistor or amplifier bias points may be set by resistive dividers from supply, 
Load currents to achieve some function may require a certain current through a resistor which cannot be achieved at lower voltages.   
Voltage swing at some node may cause clipping if voltage is reduced.


Consequently:


Resistor dividers which result in a critical voltage which should not be changed may need the top resistor reduced to maintain the voltage, whereas others where the overall voltages may be scaled down proportionately may work well with the existing resistors. 
Load resistors may need to be reduced in value if current is important or possibly increased in value if voltage drop is important, or may be OK as is [Agh!!!].