You can try staling the cache - set http client cache stale - but it probably won't fix your problem and will make all other prompts get refreshed from the Web server.
Now are you absolutely certain the file is good? I assume the URL is exactly right. Try putting the complete URL in a browser and set the default player to Windows Sound Recrder (sndrec32.exe) and check the properties.
The trace shows that audio-prompt load is reaching out to the URL, but the prompt is still in the wrong format. This command does work - although sometimes it takes a couple of goes before you see "Reload successful".
Try it on another file that you know is OK to satisfy yourself that the technique works.
But if audio-prompt load is not working, then there are no ifs or buts - the WAV file is wrong. You must double check - look at the file with a HEX editor. It can be in CCITT uLaw and still be wrong.
If you reload your router, the in-memory cache will be dropped. There is no other way.
Regards,
Geoff