How can I get Windows to install in Legacy (BIOS) boot on my mid-2012 MBP
Well I've finally found an answer: just use a Windows 8 install CD ROM with the standard 'install Windows 7 or later option' selected. Do not install Windows 8.1. It seems the problem is with the latter messing around Boot Camp normal procedures rendering audio and some other components unusable.

Where my MBP 13″ mid 2012 wouldn't complete my installation with Boot Camp before, now the installation starts in legacy via the CD, it just flows and at the end of it I get to install my version of the Boot Camp drivers found on the net and audio works fine.

What might have influenced the whole process was that I accidentally erased my whole disk, had to start from the recovery over the internet (Command + R at startup) and reinstall Yosemite and my files through my Timemachine backup). Maybe it fixes something with my old recovery and EFI partitions, but I can't know for sure.