If you want to track just few keywords, you should not look for speech recognition API or service. This task is called Keyword Spotting and it uses different algorithms than speech recognition. Speech recognition tries to find all the words that has been said and because of that it consumes way more resources than keyword spotting. Keyword spotter only tries to find few selected keywords or keyphrases. It's way simple and way less resource consuming.
The only possible solution to archive this funcitonality is to use open source package like OpenEars powered by Pocketsphinx
http://www.politepix.com/openears
Openears has Rejecto plugin that implements something similar.
Pocketsphinx itself has recently implemented open source effective keyword spotting too, but it didn't get into Openers yet. It's only available through pocketsphinx API, you need to create kws search and set the target word to look for. I hope soon this functionality will reach OpenEars too.
