if you have a speed ramp in your video then you could extend the audio in the following way the first step after you've added some speed keyframes is cutting the audio track at the point of these keyframes then slow down the part between the keyframes and create a speed ramp then move this cutoff part of the audio portion to the end of the clip and extend the audio part in the middle with the rage stretch tool and to finish it off add some audio dissolve transitions like constant power and you will get something like this
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