Hello NOKNOT,
Video Splitter has no its own intergrated AC3 decoder, so it uses the decoder installed on the PC.
By default on each computer is installed Microsoft AC3 decoder that can only be used by Windows Media Player.
That is why it is necessary to install another AC3 decoder for Video Splitter could use it.
Possibly Boilsoft has its own AC3 audio decoder that is why it works without installation of decoders I've recommended in my previous post.
After installation of this decoders it should be listed in the list of the used filters in "Tools -> Show preview filters" of Video Splitter.