With current pulseaudio, this should be easy to do for a Linux machine:

n9->pulseaudio@n9->network->pulseaudio@computer->computer

On a regular Linux machine, you could easiliy set this up from a GUI, using Avahi (zeroconf). Since harmattan does not include zeroconf, you'll have to create the appropriate sinks and sources manually through the pacmd utility. It seems like this utility is also missing from harmattan, but it's in pulseaudio-utils in the harmattan-dev repository. I will hopefully find some time to look into this in the near future, and I'll post a guide if I figure it out.

EDIT: the pacmd utility from harmattan-dev won't run without openmode.. There is a way to set up a network sink from the configuration file, but I haven't tested it yet.