Mixer still crashing when trying to adjust volume with mouse wheel.
I tracked this down to this: xfce4 -mixer crashes if you dont run the configuration tool (right click, properties) and pick your soundcard, etc.
Even if it looks like the default settings are alright you need to get to the dropdown and select a card.
There is some talk at the xfce4 mailing list about some patches for this issue, I will stay alert.
Let me know if doing this dance fixes it for you or you are facing a different bug.