Download the sample movie files and save somewhere on your local drive. Edit MainPage.xaml.cs line 14 and point it at the movie with audio. Compile/run the app and see how the playback stutters every 2 seconds when the speed changes via the timer.

Now repeat but with the sample movie that excludes an audio track, see how the speed change is seamless.

Sample movies available here:

test-movie-with-sound.mp4

test-movie-no-sound.mp4