There might be several causes leading to this situation. The following ones are just the ones we encountered most often:
Please configure your optimization plugin so that either all the scripts are aggregated/combination/concatenated (the exact terminology might be different depending on the optimization plugin you are using) or that jquery.min.js and waveplayer.min.js are excluded from the deferral
CORS policy for external files
You are trying to playback external audio files but the browser blocks the files because of a missing CORS Policy on the remote server.
Please make sure that your domain is included in the CORS policy of the remote server as an AllowedOrigin and that at least the HEAD and GET methods are listed as the AllowedMethods.
HTTPS vs HTTP
Your website is currently loading through the secure HTTPS protocol but the resource is loaded insecurely and the browser blocks it as a safety precaution.
Please go to Settings > General and make sure that the two options WordPress Address (URL) and Site Address (URL) both include the HTTPS protocol https://
It is worth noting that this is not a bug in WavePlayer. This seems to be a problem coming from a WordPress core function that returns whatever is in those two options without checking whether the website is loaded securely or not.
“403 Forbidden” error
Your file is not accessible by the browser and returns a 403 Forbidden error.
In most cases, this happens because you are mistakenly using a WooCommerce Downloadable File as a preview file, which is not possible because of how WooCommerce restricts access to its downloadable files. Please remember that Downloadable files are uploaded to a folder that is not accessible by the browser. Also, it is not enough to select a file by the Preview File button to make it accessible: if the file was previously uploaded using the Downloadable File button, that file is permanently restricted from being accessed by the browser. You should upload your preview files either from the regular Media Library or through the Preview File list.