Maybe it is just me, but a horn that starts wailing in a pressurised aircraft shortly after passing through 10,000ft is a no-brainer, especially when your ears have also been popping. Yet another give-away is a significantly noisier flight-deck.
I flew the succeeding aircraft off the 73-3 production line to the Helios airframe AND (surprise!) on one occasion also had a total failure of the pressurisation controller (same type fitted). Yes I filed an ASR. Come to mention it, I have also had pressurisation controller faults and the cabin altitude horn on DHC8s - IMHO they leave you in no doubt, whether they sound the same as the takeoff config horn or not.
I am an average airline pilot of average ability - no more experienced than the fated Captain on the Helios, and possibly less. He had issues. The pressurisation controller on 737s is far from perfect - but then none are - as with all systems, vigilance and proper procedure would have prevented this accident. We shouldn't be so hard on Mr Boeing.