then try this, this should work with the lxpanel and the xfce4-panel
Exec=/bin/sh -c 'if [ -n "$(ps -U $USER | egrep lxpanel|xfce4-panel)" ]; then /usr/lib/indicator-sound-gtk2/indicator-sound-service;else /usr/lib/$(arch)-linux-gnu/indicator-sound/indicator-sound-service;fi'
then try this, this should work with the lxpanel and the xfce4-panel
Exec=/bin/sh -c 'if [ -n "$(ps -U $USER | egrep lxpanel| xfce4-panel) " ]; then /usr/lib/ indicator- sound-gtk2/ indicator- sound-service; else /usr/lib/ $(arch) -linux- gnu/indicator- sound/indicator -sound- service; fi'