Negative Voltage Converter
In the datasheet the ICL7660 is measured with a load of up to 20 mA which I think will not suffice to power four NE5532 opamps

The NE5532 typically need about 8mA each. 

You could make a simple circuit to generate a slightly more powerful -12 V line:



Source.

Note that this negative rail isn't regulated and the voltage will drop when loaded.

Depending on your circuit, this could suffice, the opamps might have enough supply rejection.

If you want a "better" -12 V I would simply use an isolated DCDC converter and then connect its output such that you get a -12 V.

For example use an isolated DCDC converter like this.

There is a 5 V in to 12 V out isolated output model. Then you do need to add a voltage regulator to make +5 V from your +12 V as the DCDC converter is a boost converter, it need to start with less than 12 V.

This solution is not that efficient (some power is lost) but since your power consumption is low that should not be a serious issue.