Only play audio when there is nobody talking near the phone

[pserwylo]

Currently, the sound plays indefinitely and at random, but small intervals. This is good for an initial proof of concept, but it would be good to be able to listen to the microphone, and then only play sound when we haven't heart anything for a small period. This will hopefully facilitate "conversations" rather than just one way communication from the phone to the baby.

Considerations:

• When Add additional built in contacts with associated voices #2 implements additional voices, we may wish to have some audio tagged as the beginning of a sentence, to be played if the baby hasn't talked to the phone at all (or for some time). Examples of this are "Tell me about your day...", and "What are you doing now?". Others will be for responses to be played a short period of silence following the baby talking to the phone. Examples are "Oh wow", "Interesting...", etc.