Sending audio through network

What you're looking for is a sound server. These programs send sound over the network. Choices include JACK, NAS, Pulseaudio and more.

Pulseaudio is the default audio system on Ubuntu and is widely available on Unix. JACK is widely available on desktop operating systems and prides itself on its low latency. I'd try these two first. See Jack vs Pulseaudio -- how is it faster? for a short comparison.

To play music on a different computer, any of these programs would do as long as you manage to install the same program on both machines (they use incompatible protocols, though some have translation modules). But when playing movies, you may have trouble because forwarding sound over the network introduces perceptible latency. Some movie players allow you to fine-tune the alignment between audio and video, you may need to play with this setting.

As Gilles said, you're best of looking into updating your soundserver config.

You can use pulseaudio to listen to a TCP port. Be sure to check padevchooser, which is a GUI frontend to update your config.

You can find a tutorial on the archlinux wiki.

Your best bet is probably VLC/VLS, but expect some nasty problems with synchronization drift as it is hard to keep video playing here in lockstep with audio data playing there.