How to copy audio keyframes to another audio clip?

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It's exactly the same method as copying Effects. Select the audio clip with your keyframes, then select the clip(s) you want to paste them to and type Command-Shift-V (Paste Attributes from the Edit Menu). You will be presented with this dialog:

Be mindful to select whether you want the exact timing (Maintain) or have the keyframes "scaled in time" (Stretch to Fit) before clicking the Paste button.