The lobster cannot track Ranka (or Sheryl) singing anytime anywhere, because when Ranka sang Aimo on top of that hill, there was no reaction from the captured lobster.
I'd say it's via Sheryls' earrings that the lobsters can sense their singing.
example:
i) In ep2, the lobster that was hiding was activated when Ranka ran past Sheryl at the SMS HQ. Ranka must've been singing all along but the lobster only got activated when Ranka was near Sheryl.
ii)Ranka singing at Zent city, Sheryl was there too.
PS> d/ling 'Diamond Crevasse' w/o Sheryl now