A simple timer app, similar to timer-applet

alarm-clock-applet might be what you are looking for, you can save pre-sets for either alarms (specific time) or countdown timers. Two advantages over timer-applet is that several timers can run at the same time, and you can set a custom snooze time. It's in the Software Center.

Still quite buggy at the time of writing, but worth mentioning: an attempt at creating a timer that is integrated in Unity (as featured on OMG!Ubuntu!) -- teatime-unity

I've also had trouble with timers. What I settled on, finally, is the Tomboy Reminder plugin - I work in 20 minute bursts. So I just set a reminder in a Tomboy note for 20 minutes from now. When the time has elapsed, the note appears on top of everything else, impossible to ignore. If I decide I need more time, I just type in a new time for the reminder. (I do realise you were asking for a sound, and a countdown in the panel, but because I was looking for the same thing and found this to be an excellent alternative solution, I thought you might find it helpful.)