The board is aimed at the Uno-style layout. It will be blocking a couple of pins below the normal Uno portion of the mega. The board outline follows the Uno almost exactly, so I would guess that at most maybe 2 pins will be blocked.
The board itself runs of 5V, and has its own 3.3V regulators on-board (separate power supplies for digital and analogue portions). The communication pins on the dsPIC are 5V tolerant, so will run off a 5V or a 3.3V board as long as there is a power supply greater than about 3.6V to run the LDO regulators. As it stands, all the audio processing / IO is performed by the dsPIC chip, so the power of the "host" controller is pretty irrelevant. I may well be adding portions to the firmware to allow playing of audio data direct from the host controller through SPI, however if that is the main usage you want from it, and the host controller is powerful enough, then an I²S codec chip would probably be a better option. This board is aimed at off-loading the audio processing and SD card IO from the host controller on to the audio board, thus freeing up the host controller for more important tasks.