Audio problem: It is well-established that the symbiote can communicate with Eddie telepathically and others do not hear it. However, it can also speak normally and does so plenty of times; when the two are arguing in the bathroom of the prison, we see that Venom opens its mouth and move its 'lips', which would mean the woman in the stall and Detective Mulligan can hear him - which makes no sense in the scene. (00:06:30)
Audio problem: Eddie Brock is trying to find where Carnage might be headed. To draw inspiration he listens to the recording of the last time they spoke (when incidentally he was never seen retrieving the tape). However, the lines in the recording are different takes from the original scene, spoken at a brisker pace without dramatic acting pauses. (00:38:00 - 00:42:40)
Answer: It is deliberately never shown on screen, so unless it gets answered in a sequel the audience is left to speculate. Mulligan does come into direct contact with Carnage since Cletus is the person who kidnapped him. In addition, the fight between Venom and Carnage takes place all over the church with the pair constantly stabbing, slashing, and bludgeoning each other. A piece of either symbiote could be somewhere in that building. Again, though this is just speculation since it's never shown how Mulligan is infected.
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