Excellent guidance. I may have succeeded. I deleted everything and even deleted them from the Recycle BIN. Could not reboot since the wife has 30 folders open in Chrome. *%### Amazon!
Extracted all of the arduino-1.8.19-windows.zip file to C:/ and waited. A C:/Arduino/arduino-1.8.19 folder was created with 3866 files and 843 folders and 539 MB. A C:/arduino-1.8.19 folder was also created of the same mix.
I deleted the nested folder and left the C:/arduino-1.18.19 folder as my working IDE.
The unzipping process again created a C:/Users//AppData/Local/Arduino15 that was 36 MB with 10 files and 4 folders.
Copied the guestbook boards.local.txt into the C:/arduino-1.8.19/hardware/arduino/avr folder. Am not comfortable modifying the 1.8.3 Arduino platform.txt with the 1.8.5 Teensy.txt so left the Arduino there for now.
Opened the IDE with its default "sketch" and noted that there was no Teensy boards in the drop downs. Just Arduino in the menu.
Added the https://www.pjrc/teensy/package_teensy_index.json to the Board Manager URL and then searched for "Teensy" which brought me to Paul's 1.58 Install. Step 1 of 2 took 15 seconds and Step 2 of 2 took 10 minutes. I thought it might be hung up but was patient.
My C:/Users//AppData/Arduino15 folder now is 696 MB and has 8897 files in 1689 folders. Arduino15/packages/teensy/hardware/1.58.1 has a 94 KB Boards.txt file that says it is the "Solution for Arduino IDE 2.0..." but it has a crap load of teensy2 to teensy41 menu keys in it so I trust that my teensy40 is in there. The platform.txt is version 1.8.5 but does not appear to have the Audio call ins that we need for the Audio Tweaks. Again, not a comfortable coder. See "virgin" above.
Ran the "Verify" to the same code that gave me the FQBN error above and got the message below and the loader popped up.
Warning: Board arduino:avr:teensy40 doesn't define a 'build.board' preference. Auto-set to: AVR_TEENSY40
Warning: Board arduino:avr:teensy41 doesn't define a 'build.board' preference. Auto-set to: AVR_TEENSY41
Opening Teensy Loader...
teensy_size: Memory Usage on Teensy 4.0:
teensy_size: FLASH: code:120736, data:12592, headers:9004 free for files:1889284
teensy_size: RAM1: variables:19744, code:118200, padding:12872 free for local variables:373472
teensy_size: RAM2: variables:45440 free for malloc/new:478848
I did not send it to the phone to check if it loads yet. I know the chip works because I got the Blink Programs to load last week.
I believe that I stumbled my way to getting the software into my Teensy 4.0 and Audio Shield. I will load it tomorrow and see what hardware, speaker, microphone, falling/rising edge problems I have then.
Third time configuring the software is the charm!!! Gotta love it!! Thanks for all of the help getting me this far. Wedding is in 2 weeks.