Your video clip's audio is on by default; your video's soundtrack will be played at a lower volume so that your clip's audio can be heard. To turn your video clip's audio off, click on the clip -- the preview area should come up. Here, you can also edit the video clip's start and end point.
If you're having trouble hearing your video clip's audio, try increasing the volume level of your clip before importing it into your video. There are many different tools to help you do this, including Zamzar. (While on Zamzar, before pressing OK to convert your file, visit the Advanced Settings and check out "Sound volume". Select "+2" to increase the volume of your video clip when it's converted. Then, import that converted file into your Animoto video.)
For more information on how to get the best sound while recording video, be sure to check out this lesson from Vimeo Video School: Capturing Good Sound.