I know next to nothing of Premiere's AVCHD support, but if you're using DShow codecs to import them, you should try your files in graphedit first.
If I read between the lines, I gather that you regsvr32 the ax files found in PMB and expect them to work. Well, the video would, but not the audio, as Sony disables the AC-3 codec by default (probably because of licensing).
I suggest you find another DShow AC-3 decoder to make it work (AC3Filter will work in GraphEdit for instance). Once it works in GraphEdit, app supporting DShow filters should be able to deal with.
This is all assuming that Premiere CS3 works this way.
From what I understand, Adobe joined the AVCHD consortium a while ago, but didn't do anything to support the format yet, stating that it's not a pro format. I suspect they won't be able to ignore it for too long anymore with prosumer cams coming out this year.