Capturing mains AC waveform with audio recording device
Most audio circuits have a low-end cutoff frequency in the 20 Hz range. So, even at 50/60 Hz, they will have a small attenuation, and a noticeable phase shift (perhaps 10-20 degrees).
Why do you care about the waveform so much ? The AC line is quite 'dirty' and most loads don't care. Motors and (incandescent) lamps don't care. Laptops, TVs, and other loads draw power proportional to the input waveform (power factor correction) and also don't care.
You are challenge is most likely to be large V spikes (to 1000's of V ?) and may damage your recorder.