Your AVI is not suitable for editing, it is a delivery format. Use GSpot to find the codec and if it is not DV AVI type2, either get the original footage, use other software or convert to DV AVI type2.
AVI is but a "wrapper." This ARTICLE will give you much more background, plus direct you on finding out what is inside that wrapper.
It might be as simple as a setting change, but conversion will likely be the path to take.
Good luck, and let us know what G-Spot (in the article) tells you about that/those file(s).
To add to the above, the DivX/Xvid CODEC's often give one what you are getting. There are three scenarios:
1.) no Import at all
2.) Audio, but no Video
3.) Video, but no Audio
If G-Spot tells you that you have either of the above (or one of several others), then conversion, or going back to get the origianl source footage are your only options.