Sorry if this has been asked, search has rendered no joy.  How do you relocate an existing .xap file without losing the ability to import additional media?

That question doesn't make too much sense. You can move around xap files just fine, all you need to take care of is to move relative sound files with it.

I created a new XACT project in My Documents, imported some .wav's (made a sound bank etc) and saved it.  If I put a copy of this .xap file into my game's content folder, ultimately I can hear the sounds in my game.

I decided to save the XACT project file directly into my game's content folder for convenience.  I opened the XACT project and did a Save As... into my game's content folder. 

The game still runs fine, but now if I try to add more .wav files into the XACT project, XACT throws a "file not found" error; as if I've supplied it with an invalid file name.  It does this if I browse to the files with the dialog or even if I drag & drop them from windows explorer.

I can open all of these .wav's in Audacity or Premiere, so the files themselves are ok.

If I do another Save As putting the XACT project back into My Documents, I can then import .wav files without any errors.  So in this case it seems like simply moving the project somehow prevents me from being able to import new .wav files, but I don't understand that.

The XACT tool does not update the paths to the wav files to reflect the new locations, so it looks like you need to re-add the wav files to fix this. This doesn't surprise me too much given the overall UX with the XACT tool...