I personally don't have any problem with my YM sounds BUT I have 2 plugins installed that may make a difference.
First, I like the old YM sounds, so I have the [url=http://www.wackyb.co.nz/dl/Download/Yahoo_Messenger/Old_or_New_Alert_Sounds_Toggle/]Wacky B Old/New Sound Alerts Toggle[url] installed. YM overwrites it with some updates, but since it's a separate download, it's easy enough to re-apply if needed. I don't have any trouble with the volumes on them. I might mention is says in there some people don't like the new sounds as they have trouble hearing thme...
I also have the
Spoken Status/Alert from the YH Plugin Gallery. While I don't like the spoken part it does make customizing sounds for individual contacts very easy and I've always found the volume levels just fine.
I hope one of these will help. I've used the Wacky B Toggle for some time now and never had a bit of trouble with it. The YH plugin is a newer option I've tried and it seems ok, but if YM shuts down unexpectedly, you do have to re-enable your plugins which is a bit of a pain. Good luck!