When I resume from a STR with the Xfce4-mixer applet added to the taskbar, the sound is completely broken. This problem happens only with the xfce4-mixer applet, other volume control apps like gvolwheel doesn't cause this problem. I could provide more informations if needed, unfortunately I haven't found anything useful in the various logs.
Is this still an issue with version 4.9.0? If so please enable the runtime debugging mode as follows: xfce4-panel -q export PANEL_DEBUG=xfce4-mixer-plugin xfce4-panel and attach the output here.