Looks like XBMC isn't able to enumerate pulseaudio correctly. Unfortuanatly it stopps working every now and then and you have to restart pulse audio "pulsaudio -k"
1) Set channels for the daemon
To set the default channels (5.1.) for the pulseaudio daemon we can edit /etc/pulse/daemon.conf as root and edit the following lines to
default-sample-channels = 6
enable-lfe-remixing = yes
2) Set a default card profile
Alternatively we may also define a default sound card profile in /etc/pulse/default.pa (or if defined for a user session only in ~/.pulse/default.pa).
set-card-profile <index> <name>
where we need to give the <index> of the sound card (e.g. 0) and the <name>(e.g. "Digital Surround 5.1 (HDMI)") of the profile we want as default. Possible values are listed with the following command
Then use "pulseaudio -k" to restart pulseaudio