http://www.videohelp.com/tools/AC3-Filter
v2.6.0b - 2013-04-05
* FFMPEG updated to v1.1.4
* Use FFMPEG for AAC decoding (should be less crashes because faad bugs).
* SPDIF/AC3 encoder: encode 5.1 with back channels (used by flac for instance) like 5.1 with side channels (standard for ac3).
* Serbian language added (thanks to Rancher).
* Languages updated: Basque (thanks to Xabier Aramendi), Chineese (thanks to PoYang).
* Bug fixed: some languages was reset to english after restart.
* Installer: /noicons option works now
Mention it because AC3 Filter doesn't get updated all that often [last was 6/17/12], & it might be a good opportunity to get the full pack installed if you've only got it partly installed on your PC/laptop. If you don't have it installed [full or partial], AC3 Filter is really optional -- it can provide AC3, DTS audio decoding for playback *If* the software you normally use can't do that on its own.
About partial installs...
I've seen several cases where audio/video software adds just the AC3 filter itself, without all the controlling software that lets you tailor how it works, lets you often solve &/or avoid compatibility problems, and lets you uninstall AC3 Filter. If you've got just part of AC3 filter installed -- i.e. you don't have the controls in the Start Menu etc. but a search on your drive for ac3filter.* finds files -- at the least you've been shortchanged.