follow these steps:
1. mark the clip you'd like to edit
2. seperate the video from the audio file by ungrouping it (press Crlt+M)
or
3. now simply replace the old audio with a new one
If you want to change the tone of the voice go to Effects / Audio effects and pick a changer. There is one there that makes the voice low and one that makes it high. Plus CB radio and others.