Just one example, separate memory for storing said data, which when accessed in write mode (current line or write enable low voltage signal for writing to DRAM)

Just one example, separate memory for storing said data, which when accessed in write mode (current line or write enable low voltage signal for writing to DRAM) activates LED as well. Or, again if DRAM, DQ pins and monitor for voltage high. I'm sure there are a dozen of ways to do it in hardware like that. Maybe someone with more knowledge can chime in. Doing it via microphone/amp wouldn't do it, since you'd still want to use it without LED on (and not software controlled).