var stream = connection.streamEvents['streamid'];

// or use this code:
// backward compatibility
connection.streams = connection.streamEvents;
connection.numberOfConnectedUsers = 0;

if (Object.observe) {
    Object.observe(connection.streamEvents, function() {
        // for backward compatibility
        connection.streams = connection.streamEvents;
    });

    Object.observe(connection.peers, function() {
        // for backward compatibility
        connection.numberOfConnectedUsers = connection.getAllParticipants().length;
    });
}

# you can even use "getStreamById"
var stream = connection.attachStreams.getStreamById('streamid');

# to get remote stream by id
var allRemoteStreams = connection.getRemoteStreams('remote-user-id');
var stream = allRemoteStreams.getStreamByid('streamid');

connection.onstream = function(event) {
    if(event.stream.isScreen) {
        // screen stream
    }

    if(event.stream.isVideo) {
        // audio+video or video-only stream
    }

    if(event.stream.isAudio) {
        // audio-only stream
    }
};

var stream = connection.streamEvents['streamid'].stream;

stream.mute('audio'); // mute only audio tracks
stream.mute('video'); // mute only video tracks
stream.mute('both'); // mute both video and audio tracks

// or simply
stream.mute(); // mute both video and audio tracks