FAQ: How do I convert audio files into supported WAV files in Audacity?
For compatibility with Experiment Builder, audio files should be in signed 16-bit WAV format with a sampling frequency of 48 kHz or lower. Audacity, an open-source application, makes it easy to convert audio files to this format.

Download Audacity from https://www.audacityteam.org/

Converting a single file
Open Audacity and import/drag in your audio file.

If your file has a sampling frequency above 48000 (48 kHz), select all, and change the project to rate 48000

To save your edited audio file, select File > Export > Export as WAV

Finally, select WAV Signed 16-Bit PCM as the file type and save it to your PC

Multiple Audio Files - Batch Convert
If you have a large number of audio files to convert, you can import them all into a single Audacity session and batch export them in one go.
Select them all and change the Project rate (if above 48 kHZ) and select File > Export Mulitple

Set the format to WAV Signed 16-Bit PCM and define the naming of the new files (if necessary), and select export.