Is this over the PA, or from the cockpit, or somewhere else? Is this specific to an airline? Or a specific aircraft type?

I doubt it would be the middle or inner markers if they are "a few minutes" before landing. The middle marker indicates the decision point on an ILS to go missed if the runway environment is not in sight (on most ILS approaches this is 200 feet above ground, or maybe 20-ish seconds before landing). The inner marker indicates the aircraft is over the threshold, so this obviously wouldn't be it. Maybe it's the outer marker (indicates glideslope intercept location, which would be a few minutes before landing). The Wikipedia page has audio examples of each.

It could just be an indication from the pilots to the FAs to finish their inflight duties and get buckled in.