So I was playing around with a bunch of NOS LM311 chips and an ACI card with different audio sources.
What did I discover, not all LM311 chips are created equal. Even ones from the same manufacturer and date code.
The all worked with a real cassette player, but at slightly different volumes. Some didn't like iPod and iPhone audio. Some wouldn't load at all and some would put intermittent bad data into ram. This was very apparent when loading Apple the 30th. You'd get corrupted images. My guess is this is related to the fact the iPod and iPhone can't drive as much volume as a real cassette adapter. I think there will also be some variations related to your power transformers and supply components, since the LM311 uses unregulated negative voltage to the LM311.
So if you have a problem with your ACI with an iPod or iPhone, before you go crazy, swap out your LM311 and if it works don't throw out the original one. It may work in another ACI on a different board.