Deciding which stereo mix codec to install will depend on your operating system.  If you are unsure if you need a 32 or 64 bit, the video  will help.
So you've created something using one of the Dectalk Speak Windows, but now you want to record it.  Or maybe there's some audio on the internet that you want to record.  Stereo mix will let you record any sound that can be heard through you computer speakers.  Simply download and install the appropriate codec, set up Stereo Mix, then use any recording  program to record the audio.