Engines on plane during flight sometimes sound like they're slowing down, why does this happen?

The pilots are adjusting the thrust. They could be doing this for several reasons: slowing down (because they are getting too close to "red line" or ATC told them to slow down or they want to make more money and thus get to the destination later), speeding up (ATC told them to speed up, they want to get to the hotel faster), descending, climbing, etc.