The input should be capacitively coupled, so even if your mic has a DC offset it shouldn't matter. Either the capacitor is leaky or the ADC is broken. It could be a driver weirdness but that sounds unlikely.
If you don't mind losing the few bits of dynamic range, you can just remove the DC offset. A high-pass filter at 20Hz should do the trick, if your audio editor doesn't have a DC removal filter. I'd guess compressing the audio also limits the bandwidth so there is no DC.
