The video editor in Seenit has separate video and audio tracks, but the audio tracks are for music and sound effects. The sound in your videos is contained within a video track.
To expand a video track and see the sound waveform, click on the arrow next to the track title, and select the expanded view, as in the image below.
Now you can control the opacity level of your video or the volume of the sound. To do so, click the clip and select the dotted line to control video or the speaker icon for audio. Both can be seen in the next image - the green line is for video opacity and the blue line is for volume level.