1) Install smplayer (http://www.smplayer.info/en/downloads)

To install SMPlayer, just run these commands in a terminal:

$ sudo add-apt-repository ppa:rvm/smplayer
$ sudo apt-get update
$ sudo apt-get install smplayer smplayer-themes smplayer-skins

2) Convert subtitles from sub+idx to vtt for this setup

$ sudo apt-get install mono hunspell tesseract-ocr

Download Subtitle Edit from https://github.com/SubtitleEdit/subtitleedit/releases (portable version) and unzip it.

$ mono SubtitleEdit.exe

Subtitle Edit runs tesseract OCR on the video.sub file which in this case was:

$ file video.sub
video.sub: MPEG sequence, v2, program multiplex

and after conversion:

$ file video.vtt
video.vtt: UTF-8 Unicode (with BOM) text

3) Make Chrome your default WWW browser.

4) Open your video in smplayer and select Play on Chromecast from the Play menu.
(smplayer opens a short configuration menu in Chrome.)

5) Press connect to Chromecast and Play on Chromecast.