Often, people request the removal of an Amara video page which they created just to try the subtitling tool. Such a removal can be problematic. For instance if someone else has started subtitles on that page, removing it would destroy their work, so it can't be done.
So, if you just want to try the subtitling tool, you can use the Sandbox page. By default, this https://amara.org/en/videos/UnjvMd1yQrXx/info/sandbox page only shows a blank video, and this rather limits what you can try to do with the subtitling tool.
However you can use the URLs tab - https://amara.org/en/videos/UnjvMd1yQrXx/url/3247729/?tab=urls - to add the URL of a real video with audio content: see CuervoNegro's addition of http://www.youtube.com/watch?v=IQBD7_YYOCE in this URLs tab.
Once your URL has been added, hover over it and choose "Make primary": your chosen real video will now play in the sandbox and you can try the subtitling tool on it.
When you are done experimenting, go back to the URLs tab and now
You can also delete the subtitles you have done in the Sandbox from within the subtitling tool:
Your subtitles will be hidden in normal viewing, but will remain in the history of revisions. If you want them completely removed, leave me a note in a reply to this topic and I'll do it.