I stumbled across these and thought others might find them useful. Sometimes it’s hard to focus on your meditation practice when trying to keep your eye on a clock. Play this mp3 file and the sound of a bell will help you keep time.
Click the link to play (or right click to download it to your own computer):
Mindfulness of breathing, 20 minutes, bell every 5 minutes
The Wildmind Buddhist Meditation site has several more examples, ranging in length from 20 minutes to an hour.