It's easy just put an electrolytic any type between pin 4 and 8, taking care of polarity. You can take anything from 1mfd to 47mfd, I prefer 10 to 22 mfd
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If there is already a buffer electrolytic close (1-3cm) to the op amp soldering the cap is not necessary.
Sonically more important mod: if you have regulators like 7812 and 7912 or 5v types of them without Cs between the legs like in the datasheet then adding them makes the sound much better