Does anyone know what's the input offset voltage (oscillated 2.5V) to the op-amp in the circuit for? From my understanding, the input offset voltage needs to be extremely small so that output DC offset can be reduced. But for this circuit, the input offset voltage is big, therefore, it is making the output offset voltage high. Hope someone can help me!
Can you explain how DC offset at the output is an issue, because I don't think it is DC offset nor that it is an issue.
This is a microphone amplifier, microphones do not work for DC. There are even capacitors (C1 and c3) present to block the DC.
You can do the same at the output: simply connect a capacitor (of 22 uF for example) in series with the output: hey presto, your DC offset is gone.
This circuit uses a single 5 V supply. To do so the inputs of the opamp need a proper "working point" and that is set at 2.5 V by R3 and R4. The consequence of this (the single supply) is that the output will also have a constant 2.5 V DC voltage. But as mentioned, for audio signals this is a non-issue as a simple AC coupling (capacitor in series) eliminates that DC voltage.