(2) is possible because AVCHD is effectively the same format used on Blu-Ray discs, but it's a little fiddly. There are several third-party demultiplexer (demux) applications available which can separate the elementary streams from the MTS file on an AVCHD media card and give you the H.264 video and AC3/PCM audio streams as separate files, and matching multiplexer apps that will stitch the streams back together again without altering their internal data. If you can create a replacement for your audio stream which exactly matches the original in bitrate/length/timing then it should work just fine, but it's hardly a one-click workflow.
As this isn't anything to do with Adobe software and these apps are used to extract copyrighted material from Blu-Ray discs I'm not going to give links - but Google for 'AVCHD demux' and you'll find things to try.
Thanks for the responses, but I'm still not clear about what is and is not possible. I've got the full Adobe suite, so if any of the software included in the suite can automatically sync sound, please let me know.
1. Jim Simon responded that it might be possible to sync sound if both clips have identical timecode. Well, my recorded sound does not have timecodes, so does that preclude Premiere from automatically syncing external sound to internal AVCHD sound?
2. Dave Merchant suggests third-party software for (2), the demuxing/remuxing. That's sort of okay, but I'd rather stick with Premiere (if possible) or FCPX (if I have to). So, does Dave's answer mean that Premiere is not able to replace sound inside an AVCHD clip without re-encoding the video when saved?
3. Jim Curtis suggested PluralEyes. Looks good for automatic syncing. But it means another piece of software, and for $100 extra I can buy FCPX. He also says that FCPX has this function built in (I assume he means auto sound syncing).
I'm left with this (correct me if I'm wrong):
A: FCPX and PluralEyes can automatically align sound files. Premiere cannot.
B: FCPX and Premiere cannot, without re-encoding when saved, align and replace the audio in an AVCHD clip. For that I need other software.