Keynote audio clips do not complete before next slide

What do you want to happen?

And what do you expect to happen?

If you have an audio clip on the page, plus an automatic transition to the next slide, then it's up to you to set the slide timing to match the audio clip.

If your audio clip is, say, 30 seconds long, and you have a transition set to move to the next slide after 20 second, what do you think it going to happen to that last 10 seconds of audio?

If you want to hold the transition until after the audio completes then you will need to adjust your slide timers accordingly.

Alternatively, if you want the audio clip to continue playing on the next slide, copy the audio object and paste it onto the next slide, and make sure the 'Play Audio Across Slides' option is checked in the Inspector. This will allow the same audio clip to continue playing over the slide transition and not get clipped.