[theora] How to merge and cut theora videos, and how to synchronize sound?

[Lucas Malor]

Basil Mohamed Gohar wrote:
> As for the desynchronization issues with TS files, you may need a more
> recent version of ffmpeg2theora, because, if I'm not mistaken, some
> versions of ffmpeg were broken in relation to MPEG TS files.

Unluckily I deleted TS files times ago.
I think I can use oggz-rip to extract video and sound bitstreams, insert a delay to or cut the start of the oga file, and re-merge them.
The problem is: how can I manipulate in this way the oga file?