Audio Not Playing on Imported Video Files
Check for a couple things, as there's more than one thing to cause this. First, make sure sure in your preview preferences that audio is enabled. Also, check that the duration isn't set to zero (default should be 30s, set it appropriately). Then RAM Preview with 0 on the number pad. You can scrub through audio only in the timeline by ctrl+alt+dragging (Windows) or cmd+option+dragging (Mac) to double-check existence of audio. You can also preview audio only by choosing Composition > Preview > Audio Preview.

More detail can be found at these two links:

Adobe help document on previewing audio and video.

Adobe forum post about this issue.