Captivate 12 review link TTS audio keeps cutting off at the start and end for some users, not all.

This audio issue has been around for many years with several Captivate versions (both 11 and 12).

The usual reason has been that TTS voices have no audio waveform, only pure silence, just before and after the artificial voice narration.  Some users experience issues with audio cutoff because the audio player in their browser takes a split second to get the message that there is audio and therefore starts playing a fraction too late.  Some also cut off just before the audio ends because they detect the silence and shut down too early.

This issue does not happen when you have human audio recorded via a microphone because there is always at least some low level of sound before the narration begins and ends.

So the usual fix is just to record a very short (half second) of audio with your microphone, reduce the audio level even further by editing the audio clip in Captivate until you cannot hear any audio with your ears.  Then import that clip into the beginning (and if necessary, the end) of your TTS audio clips for each slide.

Try it and see if that resolves your issue.

Another solution that is now available is just to use and AI generated voice. They are usually much better quality than Captivate's TTS and can sometimes be mistaken for actual voiceover.  I find that they do not seem to suffer as much with the audio cutoff issue because they always have at least a small amount of background audio.
I have found the voices from elevenlabs very good: Free AI Voice Generator & Voice Agents Platform | ElevenLabs