How do I rid of voice oscillation when adding to binaural be

Mac OS X El Capitan
Audacity 2.1.1 (dmg)
Lame library-v3.98.2

So now I have a new problem… I am trying to drop a finished MP3 audio file on top of a binaural beats project (That I created in audacity). It makes the voice occilate and I cannot find a way to stop it happening. Does anyone know how to do it in audacity or know of another free program I can use to come these two projects without voice oscillation?

Try removing infrasound from the tracks before you combine them, to avoid a tremolo effect …