I wish I could cite some specific audio examples. I should try putting something like that together, because I see questions like this come up a lot. Not that I’m an expert in the language, but my musical background helps distinguishing the sounds lol.
I had one Japanese lady once explain it basically as this; in regular conversation you just make the “o” sound. But if she were saying the sentence slowly, for instructional purposes for example, she would sound out “wo”. One of those fun weird things. I’ve not had any 4 beat o-sounds yet but I got to do a 3 beat one with my tutor last week. It snuck up on me and I felt ridiculous.
I haz a blog http://maninjapanchannel.wordpress.com/
https://www.youtube.com/watch?v=qLQzB-1u-dg