I had it too, for the last couple days, but I was also getting a simultaneous alert message, saying I didn't have enough space, which was puzzling to me, since phone storage was at a minimum.
My SD card was full however, with MP3's, but I'd never intentionally actually sent anything other than music to the SD card.
Anyhow, I think I fixed the notification sound absence (I wasn't getting any notifications whatsoever) or it fixed itself for the time being. It turns out I hadn't been clearing out the trash or the sent folders on my HTC mail client, so that made the difference, and gave me some more space.
On the other somewhat related topic, I had also been having problems sending photo attachments via HTC email and I'm guessing that is because the SD card is full and I didn't realize that every email attachment from my HTC mail client is storing on the SD card in a folder called ".mail" which I wasn't even aware of, which then in turn filled up my SD card without my knowledge.
It's kind of weird because I never intentionally store anything on my SD card except music. I've prompted my photos to save to internal storage, and whenever I open an email attachment I just look at it in the picture viewer. I don't prompt them to save at all, much less to the SD card.
Why the heck are all these files storing themselves on my SD card? I've checked all my settings and they seem all to be ok, so I wonder why this is happening, and I guess in the meantime, I'll just have to monitor that folder and clear it out every so often?
thanks in advance for any input anyone can give