the problem with hdmi only output is that if the content is protected (hdcp) then it may not work with non-hdcp compliant devices.
also that it is digital but 3.5mm sound is analog so that needs a converter.
this device is one which should do what you want...HOWEVER..... i do not think it is hdcp compliant so it might not work with a blueray disk player (which i think ALL are hdcp).
http://www.ebay.com/itm/View-HD-Premium-HDMI-to-HDMI-SPDIF-RCA-L-R-Audio-Extractor-Converter-2013-/400623637638
this second device does the same thing (with some more options) however includes hdcp support so its virtually guaranteed to work.
http://www.ebay.com/itm/Dolby-Digital-HDMI-to-Analog-LR-RCA-Audio-Dolby-Decoder-Converter-/221156873930?pt=US_Video_Cables_Adapters&hash=item337dfa16ca
both would need:
-rca white+red to stereo 3.5mm jack adapter (you can use an adapter box and two cables or just an adapter cable)
-3.5mm stereo jack splitter
-hdmi to dvi cable for the output to the monitor.
you could then just hook the br player up to one of the stereos and to the dvi on your monitor and everything should work fine.
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yes... i know the second option is expensive ($99 seems crazy to me too) but most of the HDCP compliant audio split off adapters i found were all this expensive. if you can find something that has hdcp support and is cheaper... good luck to you however something like this is what you need.
as for the first option....it could work or it could not work. its a gamble. the lack of listed hdcp support has me worried that it will not work with your blueray player (though it would probably have worked with such things like a hdmi computer output for example)
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anyways... something like this is what you need.