AC3Filter 2.6 could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this software available. The most popular versions among the software users are 2.6, 2.5 and 2.1.
Also the software is known as "ac3filter RC 5", "AC3Filter RC1", "AC3Filter Audec CoDec". The software is included in Multimedia Tools. This free software was originally created by Alexander Vigovsky.
This free PC software is developed for Windows 2000/XP/XP Professional/Vista/7/8/10/11 environment, 32 and 64-bit versions. We recommend checking the downloaded files with any free antivirus. Commonly, this program's installer has the following filenames: ac3config.exe, ac3filter_1_63b.exe, AC3_filter_1_63b.exe, ACTMOVIE.EXE and rundll32.exe etc.
From the developer:
AC3Filter is an audio decoder that supports playback of AC3 files and DTS audio tracks. It can also process any audio track - it is possible to upmix any audio source to 6 channels. The application supports digital (SPDIF) and multichannel audio outputs.
You may want to check out more software, such as AC3 Filter, Cole2k Media - Codec Pack Standard or Spdifer, which might be related to AC3Filter.