This bug has been reported here: https://bugzilla.ubuntu.com/2803

"When hitting the play buton of the cdplayer applet, the button changes to the
pause symbol for a split seccond and then back to the play symbol and nothing
happens. This is the same for my CD (/dev/hdc) and DVD (/dev/hdd) drive.
Ejecting the media via the applet works fine. Gnome CD Player
(Applications->Multimedia->CD Player) works fine for both drives. Once the CD is
started using Gnome CD Player, the applet displays track and time, can pause it,
continue playing it and stop it. However, once stopped it fails to start the CD
again with the same behaviour as described above."

I've this problem too. These messages are displayed in the console when I click
on play:

"(cdplayer_applet2:12696): GLib-GObject-WARNING **: invalid cast from
`PanelApplet' to `GtkWindow'

(cdplayer_applet2:12696): Gtk-CRITICAL **: gtk_window_get_screen: assertion
`GTK_IS_WINDOW (window)' failed

(cdplayer_applet2:12696): GLib-GObject-WARNING **: invalid uninstantiatable type
`(null)' in cast to `GtkWindow'

(cdplayer_applet2:12696): Gtk-CRITICAL **: gtk_window_set_screen: assertion
`GTK_IS_WINDOW (window)' failed

(cdplayer_applet2:12696): GLib-GObject-WARNING **: invalid uninstantiatable type
`(null)' in cast to `GtkWindow'

(cdplayer_applet2:12696): Gtk-CRITICAL **: gtk_window_present: assertion
`GTK_IS_WINDOW (window)' failed"

after that the applet comes back in its initial state. Clicking on the "next
track" button starts the cd playing on the track 2.