Adding a guard to 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)
Moving pausing outside the playing block
Merge pull request #67 from petems/GH65-left_right_crash

Adding guards making sure a track is playing