Is there any way to check peak volume of SHOUTcast server using PHP or CGI without using the admin password?
Yes.
You need to tune into the stream and decode to raw PCM and do your calculations from there. The admin password wouldn't actually help you at all here. Of course, this only allows you to get the highest peak for the time that you listen, which doesn't tell you much.
SHOUTcast servers don't actually do anything to the encoded audio that passes through them. In fact, if you send the right data upon connection, you can send plain ol' text through the SHOUTcast server. Because of this, the server doesn't keep track of anything related to the audio at all. this is why deocding the audio is the only thing you can do here, short of getting what you need from the encoder.