Living in a resort area near the beach I would imagine that people might think that the yell came from there.....maybe a few college kids partying on the beach. Also, with the reports of the loud music and neighbors thinking a party was going on, I don't find it strange at all. After the fact it probably clicked for them. My neighbor went away just last weekend, and their teenage kids were having a loud pool party with music, and I heard a few yells, but figured it was the kids partying. Didn't think to call the police.
Also, not for nothing, it is the le's JOB to take every report seriously, especially in a possible murder investigation. To not follow up with this, not once but TWICE is irresponsible and unprofessional. My opinion.