As someone who has been designated a 'professional' audience; I dont sing or play; 2 major annoyances.
1) Other singers who talk loudly throughout everybody elses songs but expect silence when its their turn.
2) People getting up and leaving during songs.
Experienced both in the Tap & Spile at Whitby during the Moor & Coast Festival this weekend. The main culprits were regular festival goers and should have known better.