They improve Vbe-match and current-match fidelity (if
the resistors have better matching tolerance than the
transistors' Vbe). You need to establish a current across
the resistors which dwarfs the mismatch voltage (like,
if transistor population has a +/-10mV Vbe scatter,
setting a 100mV (Iset*R) value ought to improve the
mismatch 90%.
You lose a little headroom but in older +/-15V analog
(or discrete) this is tolerable.
An audio circuit doesn't care about DC as long as
it's centered and linear, right? If there's feedback
then small mismatch error in current mirrors may not
amount to anything. You'd need mismatch modeled
to say.