I followed the manual to install the Hifiberry dac+ on a Raspberry Pi B+ and it works well, I am able to play a mp3 file with mplayer, sounds great :-)
I now have the problem that I am not able to open two instances of mplayer at the same time, the error "[AO_ALSA] alsa-lib: pcm_hw.c:1293:(snd_pcm_hw_open) open '/dev/snd/pcmC0D0p' failed (-16): Device or resource busy" occurs.
If i wait until the first mplayer instance finished playing the other instance runs without a problem.
I understand that the Hifiberry does not include a hardware mixer so the man pages of ALSA told me that it will automatically activate dmix, does this work with the Hifiberry?
My (custom) program uses Qt with GStreamer, but I am testing with mplayer to get closer to the issue.
My aplay -l:
**** List of PLAYBACK Hardware Devices ****
card 0: sndrpihifiberry [snd_rpi_hifiberry_dacplus], device 0: HiFiBerry DAC+ HiFi pcm512x-hifi-0 
Subdevice #0: subdevice #0
My aplay -L:
Discard all samples (playback) or generate zero samples (capture)
Default Audio Device
Raspbian GNU/Linux 7
Thank you in advance!