I’m noticing the same thing with SD clips captured in FCP7. It appears in particular to affect long clips. If I record a short clip for a few seconds it shows up in quicktime as audio & video together. I think it relates to how FCP is closing large files. I haven’t solved the problem yet but here are some observations / workarounds.
If you open the clip in Quicktime player (and it appears to be audio-only), say Save As, choose Movie, and just save it as a new file, it doesn’t take too long and the new clip now appears in Quicktime with both audio and video.
You can do something similar with Compressor.