Sarah: What version of Premiere Pro are you using? There's a known issue with imported clips losing their audio in the November 2016 release of Premiere Pro 2017: Known issues in Premiere Pro CC 2017
From that page:
Issue: When you try to import audio files with formats like AVCHD (MTS), mp3, and mp4 using Premiere Pro CC, you could get an error message which says, "The file has an unsupported compression type." This error could also be displayed with other Adobe audio and video products including After Effects and Media Encoder.
Workaround: Clear the Media Cache Database folder. If you still see an issue, change the file directory where the AVCHD files reside and check if the issue gets resolved. Also check if this issue is related to your Creative Cloud(CC) activation. For more information, see Troubleshooting linking and importing errors with AVCHD (MTS) and MP4 files after upgrading to Premiere Pro CC.
