Adding guards making sure a track is playing

Other wise you'll get an error: 
/opt/rubies/2.0.0-p451/lib/ruby/gems/2.0.0/gems/audite-0.3.0/lib/audite.rb:88:in `samples_per_frame': undefined method `spf' for nil:NilClass (NoMethodError)

I tried to write a test for it, but there's no rakefile to run tests and I couldn't get them to work? :cry: