I bought an Angelcare Monitor that detects not only sound but also has a built in movement sensor. This is a great product that helps prevent SIDS. I am quite pleased with the way it preforms.
I invested in the deleuxe version for Baby Samantha. It detects every tiny body movement, even when Samantha is sleeping. If no movement is detected for 20 seconds, a loud alarm goes off.
There's also a built-in thermometer that will alert you of the exact room temperature of your baby's room.
The actual movement sensor is placed underneath the crib mattress, so it's not in the way. But most importantly it brings us peace of mind.
Expect to pay over $100 for this device, but I truly believe it's worth every single penny. I also think it'a a must have for premature babies.