Are the any polyphonic (multiple simultanious voices) solutions out there?
Never seen one.There is a library that will do more than one audio file but the quality is low.
here is indeed also the limit of only having one file open at once
Sorry but no. The one file only is a restriction of the SD card technology, nothing else.
You can have one Arduino command several MP3 modules via serial commands and have each module playback a different file from its SD card:http://www.mdfly.com/search.php?search_query=mp3Mix the outputs into an analog mixer, can be as simple as having outputs combined thru 10K resistors into a Computer speaker, see this schematic:
Which begs the question of hos exactly the teensy is reading the sd card. So either the teensy has and onboard solution for sd card reding or the SGTL5000 does.
Look at the schematic. Any processor has the capability of reading an SD card, it is simply an SPI protocol. The SGTL5000 has no connection to the SD card.