This bug is an old one.
I recall it here, as it would be a good time to correct it for version 2.4.
The ARI play audio voicemail window is using an URI encoding to transmitt file names.
Very often, this encoding fails, and the voicemail audio files cannot be read by ARI.
We get an error message from Firefox or IE saying that we are missing the "text/html" plugin.
This is a bluring error message. In fact the problem does not come from a missing plugin, but simply from the fact that the URL of the audio file is not correctly encoded / decoded.
We got the missing plugin error because the file name extension is not the good one, as we get a garbage audio file name after decoding the URL.
This is very simple to correct, just remove the encoding function, or use a more robust one.
To remove this encoding call, there is only two files to modify inside
/var/www/html/recordings/misc
audio.php
recording_popup.php
Two use another encoding function, it's a bit longer but should be a very small work for an experienced programmer.
I tried to contact the author of ARI without success.