Fix SIMCom 4G Mic Issue

My custom 4G board is built around a SIMCom A7672 Lase cellular module, and every subsystem except the analog microphone path is behaving. A standard two-wire electret mic that records perfectly on other hardware gives absolutely no audio input once it is connected to the SIMCom pins on this PCB.

What I already know
• The same mic works on alternate boards, so the capsule is fine.
• All PCB traces from the mic jack through the bias network and into the module show continuity and correct impedance.
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What I need from you
Pin-by-pin troubleshooting guidance, schematic/layout review, and a clear set of modifications or AT command settings that restore audio capture. I will share the schematic PDF, BOM and any scope captures you specify; you return either a corrected schematic snippet or a concise change list (component swaps, bias resistor values, layout tweaks, firmware settings).

Acceptance
The fix is considered complete once I can place a voice call through the module and hear clean mic audio on the far end, or when an AT-based recording test logs audible input above the noise floor.

Experience with SIM7600, A7672, SIM7000 or similar modules is strongly preferred, as is familiarity with electret biasing and RF-quiet audio routing. I can handle rework on my side; I just need the right diagnosis and solution.