If you're trying to figure out why your clients can't see uploaded files, please read this article.
If your files are visible, but won't play and/or the waveform doesn't show up:
99% of the time, if a file won't play it's a problem with the device or network you are using. There are a few simple steps that will fix the issue almost every time.
Try a different browser such as Google Chrome. To ensure that extensions or plugins in your browser aren't affecting your experience, try playing the files in an Incognito window (CMD+SHIFT+N on Mac)
Try listening on a different device, such as a phone or tablet. If this still doesn't work, try disabling WLAN (Wi-Fi) and using your cellular network
Last, but not least, restart your modem and router if issues persist across multiple devices on the same network