I've run into this issue when cutting from camera files. Files that are imported using "Import from Camera" AS WELL AS files that are converted using FCP 7 Log & Transfer (with the converted QTs imported as files into X) both cause the same result. No issues while editing in X, but if you export an FCP X XML and send to FCP 7 using Xto7, the sequence shows up without audio. IA is on this and I've filed a bug report with Apple.
If you convert the files to MOV using some non-Apple application (MPEG Streamclip, Adobe Prelude, etc.) and import those files into X, then the sequence in 7 shows up as expected with the proper audio.
I am guessing that both Import from Camera AND Log & Transfer embed metadata into the converted file (or alias), which triggers the issue. What appears to be the problem is that the expoorted XML represents these clips as coumpound clips, which causes problems in the translation.
So for now, I would recommend avoiding Import from Camera or Log & Transfer for anything being edited that has to go out to another application.
This appears to be isolated to the C300 media. I just finished some simple testing using DVCPRO HD and AVC-Intra at various frame rates/sizes via both Import From Camera and FCP7 L&T and the translation is fine. The Xto7 trip back to FCP7 results in a timeline with audio and video intact. Seems like it might be a problem then with the C300 import plug-in.