Audiophile here. They may not have been loud enough for a microphone to pick them up. You also have to consider dynamic range of the audio recording. Digital mediums have a set dynamic range based on the codec and other things. This is the metric used to measure how loud and quiet the recorded sounds can be. (Probably not the best way to define that) It means something loud in the recording can cause something quiet to not get picked up at all if the dynamic range isn't wide enough
Potential Lip reading software
https://github.com/SARIT42/lipsyncr
Im on my phone so I cant try it.