This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
In the late 1980s, it was common for mid-range audio to use
discrete transistors and a few carefully placed op amps. In
the 2000s, integrated circuits are much more sophisticated
and highly integrated. The idea of swapping out an inferior
op-amp for a better part as an easy way of improving sound is
far less meaningful today than it was in the 1980s.
There are many good op amps available today. Some are
engineered for use in audio. If you want to build something
for yourself, such as a filter or buffer, select a quality
op-amp that is meant for audio use. Also, pay careful attention
to the power supplies and grounding. Remember that all op-amp
circuits process signals with respect to ground, whether they
have a ground terminal or not.
But if you have a modern piece of equipment, don't waste your
time trying to replace the op amps in it with better parts.
You may make things worse, rather than better.
As an alternative, you could consider replacing ceramic or
electrolytic capacitors in the audio paths with quality film
capacitors. This is a safer idea and more likely to improve
the sound. For supply bypassing, ceramic capacitors are OK,
but they are bad if used in between stages or as part of a
filter or equalization network. Electrolytic capacitors
are also poor if used in the signal path. You can improve
the sound by adding a large value film capacitor in parallel
with the existing electrolytic capacitor.