Find which browser plugin is used to play ?

There is no direct one way to know, you have to examine the webpage, see if there are any visual indicators of what plugin is being used. I didn't see any, so I looked briefly at the HTML source, that didn't help (I didn't look in the javascript files which would have told me). Then I looked at Activity Monitor, sorted by CPU Usage. That showed the Flash plugin process using the most CPU, and killing it stops the audio, so it looks like for the stations I was playing, the website was using Flash. But I also saw Silverlight so I it might be loading the audio using whatever plugin can play it best on demand.