Hi. Keyframes are a good way of achieving the effect you desire.
However, Keyframes can be slightly baffling/overwhelming if you are new to Power Director.
Possibly an easier (but more time consuming) way is to split the music track so you can turn the volume of the music down when you are talking.
If you split the video clip too, then you can even turn the volume of the video up when you are talking.
If you go for the splitting method, you should do this last, after you have finished editing the video.
Otherwise, if you add anything else on the tracks it will no longer be in sync.