Louise,
Very briefly, on the subject of urgent and non-urgent warnings, I would say that unless you are an Instrument Flight Rules operator (eg North Sea rig support) or a mil/police NVG/FLIR flier, GPW is not a major concern for most heli operators. Most other heli operations involve continuous deliberate close proximity to the ground anyway; warnings would be superfluous and counter-productive. Mil operators would probably count Missile Approach Warnings or Radar Warning Receiver indications of radar tracking or guidance as urgent. All operators at low level would consider wires to be the real knicker-gripper, but I've not yet seen/heard an alternative to the current warning system usually fitted (a terrified crewmember screaming at the top of his/her lungs - very effective but expensive to fit/replace). Fire warnings deserve suitable audio backup. Rather less urgent would be things like low-alt/low airspeed warnings for aircraft with retractable undercarriage. Low urgency, with no requirement for audio, would cover just about everything from cabin-door open in aircraft with speed limits for that kind of thing to low fuel.
Serial tasks? Don't know - never really put much conscious thought into them. I'll have to go and ask my mate the test pilot - they like to look at and analyse all that kind of stuff.
Cheers