I have (just now, again, only to be sure) tested the flash version and the HTML5 version right next to each other. The new version definitely is not louder. Are you sure your mixer setting don't have the Flash volume lowered, compared to the rest?
Update: Tim Stone just noticed that jPlayer silently changed their volume range numbers, so it would indeed have been louder (which wouldn't make a difference between Flash and HTML, but between before and after). I have adjusted the code accordingly now. Please let me know if it's now back to what you expect (in which case your "icicle" analogy is a faaaar stretch, the difference is noticable, but certainly not orders of magnitude) so I can tag this status-completed, or if you still have bloodcurdling pops or dings, in which case this would be status-norepro.
Oh, and regarding the discussion about the sound choice (mainly happening in the comments on your question), I'll say what I've said time and time again: Anything that is not constructive is in vain. Opinions on particular sounds are more subjective than on Jarrod's toe socks, so to change a notification sound, you have to a) offer an alternative, not just "the current one is bad, give us another one", and b) there has to be reasonable agreement that the new choice isn't worse than the old one. In such a case, we're happy to change notification sounds. The sounds for the no-longer-existing sites are still there, so you even have some suggestions to start with (1, 2, 3, 4).