If you are talking about the "tremolo" type sound on the speaker's voice, that could be caused by a defective time base in the recorder. Was it set for external sync mode? It could happen over a very narrow range of temperature. Repairing that in post will not be easy. Your issue is similar to wow and flutter. Google techniques for removing wow and flutter. If you somehow? can map the frequency variation and then create and add a tone with that variation and then use "autotune" to remove the variation, then remove the tone? Good luck. Mark
A thought, is there any "hum" or other steady tone in the recording that can be used as a reference to help "undo" the wow and flutter?