Windows

For Windows please use CCCP. Before installing make sure to uninstall any other codec packs (like K-Lite, ACE, etc) and/or individual codecs (VobSub, FFDShow, etc) that you may have installed before. And make sure to use one of the players that come bundled with CCCP.

If you are having any problems go into CCCP’s Settings application, check Reset All Settings and Re-Register Filters and confirm with Apply. If that won’t help please head over to CCCP’s forum where you will be offered help.

Furthermore if you are experiencing audio and video desync issues you might want to take a look at CoreAVC.

Mac OS

Use either MPlayer or QuickTime with Perian.

Linux

Use MPlayer.

Some Helpful Links

Softsub Playback
H.264 and MPlayer Playback Speed-up
How to Watch Fansubs with a Mac

A Word on mplayer usage with 5.1 FLAC audio
[08:53] Hey, there’s something concerning mplayer playback that I just discussed with some mplayer devs over.
[08:54] It seems that mplayer doesn’t have proper downmixing for the FLAC codec yet. They’re looking into it, but…
[08:56] Basically, it means people who try to view VOTOMS Phantom Arc on mplayer with less than the full number of channels are currently only going to hear two.
[08:56] Until mplayer adds FLAC downmixing support, the devs recommended the following flags be added to the mplayer command line:
[08:56] -af pan=2:1:0:0:1:1:0:0:1:0.5:0.5:1:1
[08:57] Anyone without the full number of channels should then be able to hear all channels of the audio.
[08:57] Someone on the comments here: http://evaforum.com/formulasub/?p=596 mentioned having this problem. You may want to mention these settings on your playback page.
[08:59] I’m telling you this because you guys disabled your comments, making me unable to just tell the guy…And I’m sure other people are going to have these issues with multichannel FLAC audio until the problem’s resolved.
[08:59] So, heads-up, hope this helps.