I haven't tried it mainly because I only hardmod and use modded bioses but here it is and I'm hoping someone can give it a go and get back with there results whether they were successful or not so I can either smile or tweak the idea a bit further.I'll come back with a drawing but until then, here is the theory..
1st off you "must" have the LED change colour when XBMC boots from the colour it is while the flubber sound is being produced, this is the trigger, that 1st colour. Let me explain. If your Xbox, when you start it via the start button has the LED colour as green, you need to set the LED colour when XBMC to red using the XBMC settings to do this. You wire in an opto coupler so it is powered off the green LED supply meaning when the green LED is on the opto coupler is on. The output of the opto coupler is used to power a small 5 volt relay and it's contacts are hooked up to open circuit the audio common wires available at the video output plug. What this will do is "mute" the sound on startup until the green LED changes colour and switches off the relay thus allowing the audio common to be reconnected and sound.
The idea can be altered to work with machines that use orange for the softmod colour but this is the easiest way to explain the theory using green and red as the colours. I want to do some testing myself when I'm Xboxing next to see if there is a better "trigger point" for the relay like the " DVD read enable" signal or any other trigger point someone can think of that is only active after the flubber sound is finished or when XBMC starts. The disabling of the sound using the audio common wire is right, we just need to get the trigger for the relay perfected if possible and then that should finally enable you softmod guys to get rid of that horrible, loud startup sound once and for all. Ow, if you use digital or coax sound, you would have the relay "break" the digital or coax common line instead of the normal audio wire.
