[Task] get input volume display working smoothly

• ideally low computational effort
• extra ideally, allowing passthrough to other recording apps
• max input level on MVX2U is +7 dBV (about 9 dBU, 2.2 VRMS, 6.3 Vp-p)
  • ten segment display showing dBU
  • two red at 7 and 9
  • three yellow at 1 and 4
  • green: -24, -18, -12, -8, -4, -1