I posed this question in Opera's forums to no avail. Hopefully someone here can help me.
The first, obvious question is, does Opera actually use a different mechanism to play sounds generated in HTML versus those generated by the UI (via the Preferences)? What could possibly let the sound play one way but not the other? If it helps, pointing the browser at html5test.com reports that the audio tag is supported by Opera on both Windows and Linux, but on Linux, none of the audio codecs show up as supported, while on Windows, several, including PCM, show support.
Other info - when I reference the file via HTTP, its type shows up as audio/x-wav, and in Opera's preferences, under Advanced:Downloads, this type is set to "Open with Opera". If I set this instead to play using ossplay, the audio will play, but there are other weird consequences to doing it this way, so it's not the ideal approach.
Clearly TC can play the audio, and Opera can play the audio if it wants to, but something is gumming up the works when I try to play it from a web page. Has anybody else experienced a problem like this? Any ideas for things I can try? If you can make Opera play a .wav file from the URL bar in your TC install, can you please tell me a bit about how you have it set up?