High Sierra doesn’t recognize AVCHD files and Premiere 2018 makes it worse

[Bart Vanhaeren]

AVCHD media (including the .MTS files contained in there) use AC3 audio tracks.
Check if you have an older A52Codec.component installed in /Library/Audio/Plug-Ins/Components. Remove it as AC3/A52 codecs are build-in to macOS nowadays. After rebooting, AVCHD import should work fine again.

The third party software Perian installes this incompatible version of an AC3 codec in the components folder, so uninstalling these might have the same effect.