Why a cell phone needs HDMI is beyond me...but I digress.
First off HDMI is a digital signal and most likely your input to your speakers is analog. So, what you would need is a converter, something like the GefenTV HDMI to Composite Scaler
. That will run you about $300 plus any associated splitters you would need.
UPDATE: You can check out some of these HDMI D/A converters: http://www.nextag.com/hdmi-to-analog-audio-converter/products-html
However, for your setup, you will probably have to get an HDMI splitter so one signal can go to your monitor and the other to the D/A converter. Or...
How important is it to you? because now we're talkin' $$