Was at work today so late in. To add to JKCalhoun's post, magnetic recording is pretty non-linear with a marked threshold. So a 'bias' signal strong enough to overcome the threshold was added to the current from the amplifier and passed through the recording head. That extra signal could be just a dc current as JKCalhoun mentioned, or a high frequency (mid tens of KHz, well above human hearing) current. See [1] for the details. I think that the 'dc rumble' from the magnetic grain is audible on the trailer recording in the OA, but it could just be noise from the bag the tape recorder was hidden in.
The microphone in the OA pictures looks pretty basic as well, and perhaps intended for voice recording mainly. So we have limited high frequencies from comparatively slow tape speed, rumble from the dc bias, and a microphone that perhaps isn't catching low frequencies.