I have released version 1.1 of the Wordselect question type that addresses this issue.
Here is the new entry into the releasenotes.md file
Version 1.1 of the Moodle WordSelect question type by Marcus Green. July 2017
Fixed a bug that prevented audio files working correctly in the question text. Thanks to Matthias Giger for reporting this. Fixed an issue that prevented any files being inserted into the introduction area. Improved phpdoc of the source code.