Using 37.0.2178.32 now, problem still there.

Same thing here, Opera 36.0.2130.65 and OS X 10.11.4: on Opera lunching, small audio burst, less than a second.

So, I installed Audio Hijack to record the sound and post it here, but when Audio Hijack is running, Opera doesn't make the burst anymore: the audio file is dead silence! My hunch is that it's related to the "Instant On" thingie installed with Audio Hijack.