I believe the watch app for Shazam is using the microphone from your phone to 'listen' to the song you're hearing (as per their website, it's been saying that since the app came out). Since my phone is buried in my pocket most of the time, I've had pretty bad experience using this app from the watch, and generally found out that in the end I had to pull the phone out of the pocket and point its microphone towards the sound source.