My experience thus far on ALF502/LF507. Of all the bird strikes where the engine was involved only one was heard. Several were discerned during walk-around aafter landing and smelling the KFC odour approaching the engine.
In one instance at a remote location engineering authorised an engine run, on completion of which, the remains of several smallish birds passed further through the compression stages, bogged the engine down, and an overtemp resulted.
The only engine strike I heard was a large Kite (hawk) going down #3 or 4, though I didn't know it had hit then engine at the time. The loudest bang I've ever heard in an aeroplane, one fan tip bent forward and I'd hate to think what might have happened during the ensuing 3.5 hr flight had we not rejected.
To answer Yon's question, I don't think there's a hard and fast rule. Had I hit the kite above V1 and heard the bang, I think I'd have burned to LWT and returned for landing.
Had I departed the remote location where the over temp occurred, I'd probably continue and monitor the engines closely, in the absence of some positive indicator of an ingestion.
As an aside I'm now operating aft mounted BR715s. I doubt one would hear anything abnormal with them from the flight deck, short of self destruction or stopping.