Sometimes the sound doesn't play automatically for the audio exercises. Hitting Ctrl + space bar should make it play (or click the speaker button on the screen).
i'm guessing this is due to Mdudu being class 1, it/he would be -m-/-mw-, the -mw- infix is used when the verb stem begins with a vowel...hence,
tu-na-mw-ogop-a
make sense?