25 sensors

5 rows × 5 columns

pins (`see SparkFun`_)::

  14
  23

1. phototransistor collector, to pull-up for row
2. phototransistor emitter, to ground for column
3. led anode, to limiter for row
4. led cathode, to ground for column

so…

::

   aM aM aM aM aM cN cN cN cN cN
   14 14 14 14 14 14 14 14 14 14 …
   23 23 23 23 23 23 23 23 23 23 …
   Mb Mb Mb Mb Mb Nd Nd Nd Nd Nd


      /---------- input1
   a -+-10kΩ--+- output1
   b ---220Ω--/

      /---------- input2
   c -+-10kΩ--+- output2
   d ---220Ω--/

   …
   …

   M ------------ output6
   N ------------ output7


.. `see SparkFun`:
   https://learn.sparkfun.com/tutorials/qrd1114-optical-detector-hookup-guide/all


amplifier
https://learn.adafruit.com/stereo-3-7w-class-d-audio-amplifier/assembly
https://forum.arduino.cc/index.php?topic=269972.0
https://www.hackster.io/julianfschroeter/stream-your-audio-on-the-esp32-2e4661