Hi james,
after some testing, it seems as if the wav-importer has problems with 24bps-wav-files. 16bps files are fine.
So far I was not able to fix this, and there is already a bug report on the problem: https://github.com/Gianclgar/GDScriptAudioImport/issues/3
I'll try to find a solution. In the meantime, you could bulk convert all 24bps files to 16bps as a workaround.