Hi Tee,
I think that your sensors are so close together that a loud sound from a nearby machine will trigger the local sensor when the sensor is sensitive enough to trigger with a low local sound.
Don't make the microphone directional with a solid tube, it will resonate. Use a tube made of thick foam rubber with dense stuff (a plastic sewer-pipe would work well) around its outside. Put the mic in the tube near the distant end with lots of foam behind it. Then the mic will be directional at high frequencies if the tube is long enough.
The parts around where the steel balls drop will reflect sound, so should be well-padded.
Of course, the activity you want to detect is audio, so will be interfered with by any other audio like car horns or planes flying over.
The electronic circuits that you need are a preamp for the mic, audio filter and a rectifier/filter that will trigger your 555 timer.
The mic that you posted is awful darn cheap so might be a factory-reject and if they work, will probably all be different. Get good prime mics from Digikey or Newarkinone (Farnell). One of these suppliers has 30 mics to choose from.
Good luck.