so this is what I use to determine if my receiver is muted, then send out the appropriate commands..

var e = CF.extender[10];
if( e == null )
{
Diagnostics.log("Extender 0 is not defined");
}
else
{
var s = e.serial[0];
if( s == null )
{
Diagnostics.log("Extender 0 is not a serial extender");
}
else
{
s.bitrate = 9600;
s.databits = 8;
s.parity = 0; // None
s.stopbits = 1;
}
}
var status = s.match('MU?\r','\r',1000);

if (status.indexOf('MUOFF') > -1)
{
s.send("MUON\r");
}
else
{
s.send("MUOFF\r");
}