Default, audio comes over audio jack, but i wanted to route over hdmi, because i have an AV-Receiver with a high-quality audio engine which should do the transfer from digital to analog signal.
Successfully tested it with Lubuntu 3.1.1 and Bananian.

First, install pulseaudio
apt-get install pulseaudio

Configure your /etc/pulse/default.pa. Add the following line at the end:
load-module module-hal-detect
It enables pulseaudio to route sound to hdmi.

At the end, edit your /boot/uEnv.txt and switch from hdmi.audio 0 to 1:
disp.screen0_output_mode=EDID:1280x720p50 hdmi.audio=EDID:1
How to get there?

#if /mnt doesn't exist, create it by
mkdir /mnt
#type the following, to see, where boot partition is mounted:
blkid
#you should see something like /dev/mmcblk0p1. Now mount it:
mount /dev/mmcblk0p1 /mnt
#now you can edit uEnv.txt by
gedit /mnt/uEnv.txt