Is the audio and video in one clip or separate audio and video clips? If they are both in the same clip, have you tried keyframing with time remap?
If they are separate clips, join them with command-L, then export them and re-import them and then try to apply keyframes to time remap.
Once I wanted to slow down an audio-only clip, but I couldn’t affect a speed change. So I joined the audio with a piece of video, exported that clip and re-imported. Then I slowed down the clip which worked. Then I separated the audio and video and used the slowed down audio. (When I tried to slow down the joined clip without exporting it didn’t work.) Hope this helps.
John
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