I have a pair of .idx/.sub subtitle files with multiple languages and I need to convert them to single-language .srt files because neither Totem nor Gnome Mplayer can show that type of subtitles.
The Avidemux way is very handy.
Very good tutorial here: http://en.flossmanuals.net/Avidemux/ExtractingDVDSubtitles/
Works very good for me, if you have more questions, please ask
idx/sub are called VobSub file and are bitmap images + timecode that are overlay on top of the video during playback.
A srt file is a simple text file with timecode that is then render on the video.
This means that to be able to convert them to text string you need to do some OCR, Optical character recognition. Sadly, most OCR solutions don't support idx/sub.
At this point, the best solution I have for you is called SubRip (see also official site) and it's Windows software only. It tends to run well in Wine, so you should be able to use it in Ubuntu (without Windows). You can also use Avidemux according to this guide.
Both will present you the bitmap image from the idx/sub, you type what you see and it will build the corresponding srt file. Doom9 has a nice guide for SubRip.
You can also burn in the subtitle to the video file, Handbrake will be able to do that.