I JUST had the same problem the other day. I've had my phone for years now, and this was the first time I can recall of having issues keeping phone volume (not speaker volume) up. If I used the on-screen phone volume control set all the way up, the next time I looked at the volume, it was all the way back down (while speaker volume remained up.) Kept trying over and over with the same results.

I played with it a while, and I think I did a soft reset, and things worked again. Some kind of bug that only occurs in May of '09 maybe?

Kalel