What is ProjectM?
ProjectM is an open source music visualizer based on the Milkdrop plugin for Windows/Winamp. It now has a Qt GUI that can visualize your audio output through either JACK or PulseAudio, in addition to a libvisual component. This wiki currently focuses on the PulseAudio standalone GUI.
The binaries for ProjectM are not currently included in Arch. However you have two options for obtaining the packages. Either build Template:Package AUR from AUR yourself or you can get them from an unofficial repository. Currently packages are being built by Luciferin and are on the compiz-fusion-git repository.
Edit /etc/pacman.conf as root and add the following repositories to the bottom:
[compiz-fusion] Server = http://compiz.dreamz-box.de/i686
Save and Exit.
To synchronize the newly added repository and install the latest package group for PulseAudio:
# pacman -Syy projectm-pulseaudio
Once everything is install just do the following from the command line:
The first PulseAudio sound output device is used from the list shown by the command
If this is not the device you want to use, the easiest thing to do is to disable (set Profile: Off) the other devices in your Sound Settings. There does not seem to be a way to force a different device via command line option. Neither does the pacmd command set-default-source seem to make a difference.
Some information on usage and configuration can be found in the ProjectM FAQ
Otherwise a good place to ask question are the projectM help forums on Sourceforge.