Is a 2N3906 an appropriate replacement for a BC158(B) in audio amp circuit?
This transistor is biased DC around 1-2 mA collector current.


Suppose HFE is at the low-end: 125...then DC base current is about 9.7 uA.
Suppose HFE is at the high-end: 500...then DC base current is about 2.4 uA.


This base current flows through base bias equivalent resistance of about 85k ohms. So at one HFE extreme compared to the other HFE extreme, DC bias would change only 0.625 V at T601. That's trivial compared to the 19V at transistor's emitter.
Since open-loop gain is very much higher than closed-loop gain, almost all transistor non-linearities are removed. So gain variations are removed too, because overall amplifier gain is set by resistor ratio of R608/R607.

T601 is not pushed anywhere near its operating envelope limits. 2N3906 is a decent substitute in this circuit.