Forking each sox command to the background causes samples to play non-deterministically and step over each other. Here's a better way:

    # Define notes for major seventh chord starting on A 440
    A=440
    C_SHARP=554.37
    E=659.25
    G_SHARP=830.61

    case "$layout" in
    Finnish)
      kitty +kitten themes --reload-in=all Apprentice &
      play "|sox -np synth 0.2 sine $A" \
           "|sox -np synth 0.2 sine $C_SHARP" \
           "|sox -np synth 0.2 sine $E" \
           "|sox -np synth 0.2 sine $G_SHARP" \
           vol -30dB 2>&- &
      ;;
    'English (US)')
      kitty +kitten themes --reload-in=all Default &
      play "|sox -np synth 0.2 sine $G_SHARP" \
           "|sox -np synth 0.2 sine $E" \
           "|sox -np synth 0.2 sine $C_SHARP" \
           "|sox -np synth 0.2 sine $A" \
           vol -30dB 2>&- &
      ;;
    *) echo >&2 'Invalid $layout - must be "Finnish" or "English (US)"'; exit 1
    esac