If I add more than one sound (I use mp3 files), only the first one is showing. If I add second file after the first one, it's not showing it or any other content after that sound. I don't think it's the problem with the template because I've tried all of them, even made a custom one, and the result is the same.
If it was just one template I would say that it must be a very old template that couldn't handle multiple MP3s. But if its all templates than it might be a case of "double filtering." Where two media plugins try to handle the same media file, basically causing the sky to fall in and hell to freeze over.
Try turning off Multimedia Plugins filter temporarily. Does that solve it? If so turn it back on, and disable handling of the mp3 file extension by the media player that Multi Media Plugins filter is using. Or move VideoEasy filter higher in the order of filters at : site admin -> plugins -> filters --> manage filters