There are lots of reasons why this might happen, over-compression (see more info on compression here: Improve Or Ruin Your Audio With an Effect Called Compression and Vocal Compression Using Reaper’s ReaComp Effect Plugin), wrong kind of mic for that voice, too much reverb effect, etc. The good news is that you can use free sound editing software to fix this! You’ll be listening to the recording, and everything will sound great, until the speaker/singer utters the phrase “she sells seashells by the seashore,” and pierces your eardrums. The word "sibilance" refers to a sharp, biting hissing sound that happens whenever the recorded voice utters certain consonant sounds, especially the “S” sound (or sh, ch, etc.).
If you didn’t already know what sibilance meant, my odd spelling above may have clued you in. One common problem with recording the human voice is sssssomething called "sibilance."