I found a tutorial to enable the HifiBerry DAC+ on Volumio here: http://volumio.org/forum/volumio-for-raspberry-model-t1574-10.html
The board as such seems to work: I see the green LED goes on on boot, or when I press "play" on the web frontend.
However, there is no sound, and I see a message on the console: "bmc2708-dmaengine: DMA transfer could not be terminated". A bit later it (sometimes) complains about "4096 KiB atomic DMA coherent pool is too small!".
Over the headphone jack, or via HDMI I could play sound (before I modified the config files for the DAC+), so the basic installation seems to be ok.
Does someone have an idea what could be wrong?
DAC+ something broken or user error?