I use the Greddy Type RS bov. I am running 15 p.s.i. and it has a nice aggressive but not too loud sound. Some say the oem bov is better but who knows for sure if that is actually factual. The fastest MR2's don't use the oem bov.
I don't have my bov vented but the following link sounds very similar to mine even though it is vented to the atmosphere. Here's a link:
http://www.gscdownloads.com/greddyrsbov.wmv