Accordingly to this bug report (https://bugs.launchpad.net/ubuntu/+source/checkbox/+bug/978895) the correct plugin to use is autoaudiosink:
gconftool --set /apps/mail-notification/sounds/play-command --type string \ "exec gst-launch-1.0 filesrc location=%file ! decodebin ! audioconvert ! autoaudiosink >/dev/null 2>&1"
Accordingly to this bug report (https:/ /bugs.launchpad .net/ubuntu/ +source/ checkbox/ +bug/978895) the correct plugin to use is autoaudiosink:
gconftool --set /apps/mail- notification/ sounds/ play-command --type string \
"exec gst-launch-1.0 filesrc location=%file ! decodebin ! audioconvert ! autoaudiosink >/dev/null 2>&1"