B737 Captain reported receiving Airspeed Low aural alert during approach. Crew increased speed and continued to an uneventful landing.

Narrative

I was the Captain and PM in a 737 from ZZZ1-ZZZ. It was night time arrival in IMC with the ceiling and visibility at 400 feet and 1 ½ miles in light rain. The descent was uneventful and ZZZ Approach gave us vectors to the ILS XX L. We were assigned 210 knots at the time of the approach clearance (and we were told to descend from 4000 to 3000 to intercept the localizer and glideslope) and we were a few miles from ZZZZZ at this point in time. We saw that we were fairly close to the aircraft in front of us (3 miles if I recall) and the PF called for Flaps 5 and he selected 170 knots in the MCP. The autopilot and autothrottles were on at this time. ZZZ Approach then told us to slow to our final approach speed due to our proximity to aircraft ahead of us (and this is sooner than usual for ZZZ). The PF immediately spun the MCP speed selector down to 146 knots and simultaneously called for gear down; flaps 15." I selected gear down and flaps 15 as commanded and we were at 3000 at this point in time if I recall correctly. Shortly thereafter we heard an "AIRSPEED LOW; AIRSPEED LOW" aural alert. I looked up at the PFD and saw that we were at 146 knots and roughly 4 knots below the Flaps 15 minimum speed bug on the speed tape (and the airspeed box was flashing as well). We were in an Undesired Aircraft State (UAS). I immediately announced that we needed to be at Flaps 30 and that we were slow for the current flap setting of 15. The PF correctly followed the upset recovery procedure; disconnected the autopilot and autothrottles; and then applied thrust while I selected flaps 30. This very quickly corrected the UAS. The glideslope then captured; I set touchdown zone; and the PF continued to hand fly the approach to an uneventful landing. The aural alert was startling and shocking as I was very surprised to hear it but as soon as I looked up I recognized what had happened. Fortunately; we were able to quickly recover and land uneventfully. Once at the gate we debriefed our flight and arrival including; of course; the errors that we made to cause the UAS. The PF believed in his mind that he did call for flaps 30 shortly after calling for "gear down; flaps 15." I never heard it if he did call for it. I do recall being surprised that he spun the MCP speed selector all the way to 146 since that was below the selected flap setting. While it is only a technique (and not an SOP); I never move the MCP speed selector below minimum flap speed for the flap setting that is selected. Had this technique been employed; it would have prevented this UAS and trapped this error. Unfortunately; it took the aural alert to trap the error and we even missed the 10 seconds of flashing on the airspeed indicator. I did a horrible job performing my duties as the Pilot Monitoring as I should have caught that we were going below the minimum flaps 15 speed of approximately 150 knots. I could have then told the PF to increase his airspeed or call for flaps 30 - either of which would have prevented the UAS. My situational awareness was not where it needed to be and I honestly don't know what I was focused on or where the distraction was at the time as I felt fairly dialed in given the night IMC situation as well as our proximity to the plane in front of us. I also think the PF could have done a better job of being situationally aware and if he had thought he called for flaps 30 and saw that the flap handle never moved then the UAS wouldn't have happened as he could have then called for flaps 30 in time to prevent the UAS (or simply not slowed down below the minimum speed for flaps 15). I do believe we did a good job of managing the error and not letting it further distract us and cause a bigger issue for the safety of the flight. The aircraft ultimately configured properly; the landing checklist was completed; the approach was stable from that point forward; and the landing was uneventful."