Any form of 'Streaming' can be problematic. You will just have to accept it as a natural occurrence and hope that it is a 'one-off': until the next time.
I have an android box, which is similar yet different to the firestick.
I occasionally suffer from the same problem with the picture and sound being out of sync, but on the box there's an option to re-sync the sound - if I remember correctly, I think you can do it in increments of 0.025 seconds - trial and error needed to get it right.
I'd assume there's something similar on a firestick??