I’ve never worked with AVCHD, but with other formats in L&T in the Import Settings tab next to the audio select buttons you’ll see a symbol. Clicking it will designate the transferred audio as stereo or mono.
A way of dealing with this if you have a lot of clips already in a sequence is to change the audio of the first clip to the way you want it, then copy it (Cmd-C), select all the following clips and Paste Attributes (Opt-V) and in the audio choices check Levels & Pans, click OL and the change will be applied.
John
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