Sound

USB sound adapter

Sweex SC016 7.1 external usb sound card.

This was supplied by Amazon UK with a driver CDROM. It was automatically detected and worked with Windows8 and Ubuntu without the need to install any drivers. The supplied usb cable is about 150cm long. The six audio channels (5.1) are output from three 3.5mm stereo jacks. The sound quality seems acceptable.

It takes some fiddling to get it working in Linux, mainly due, I think, to the convoluted tangle that is ALSA and PulseAudio, but one or more of these have always done the trick so far:

Select 'CM106 Like Sound Device Analoue Surround 5.1' in PulseAudio Volume Control
Select USB Sound Device (Alsa mixer) in Alsa mixer
Select 'ALSA audio output' > 'USB Sound Device, USB Audio 5.1 Surround output ...' in VLC player audio preferences
Unplug and re-plug usb adapter
Re-boot

Once it's working, it seems to stay working (so far).

Raspberry Pi

The only video player that works on the Raspberry Pi is omxplayer.
omxplayer will only output sound on the hdmi port. Several requests have been made in the XBMC and Raspberry Pi fora for support for usb sound adapters, but it appears that this will be hard to do. 5.1 sound is probably output via hdmi, but I haven't been able to test this.

Because a PC or laptop is always required for the control program, it doesn't seem to be a problem to use this to output the sound.

Installation video files

A guide movie with the full 5.1 soundtrack was made from the five screen movies. Here's a still:

The picture is lightweight, and the movie can run in the control computer as the primary sound source.

Other approaches to sound

A more general solution might be worth pursuing. Each screen could have its own loudspeaker which would mainly play dialogue being spoken by characters appearing on the screen, and sync effects such as the slamming of doors. Each video file would have a single sound track, which would of course be automatically in sync with its picture.

Music and atmosphere could be played separately through a surround sound system, from a guide movie playing on the control computer. This approach would be scalable to any number of screens.