Using Eclipse to develop for teensy 4, I found the three small omissions below. The are not errors, but fixing them stops the compiler from complaining:

file "analyze_peak.h" :
add "virtual ~AudioAnalyzePeak() {}" after it's constructor

file "mixer.h" :
add "virtual ~AudioMixer4() {}" after it's constructor
add "virtual ~AudioAmplifier() {}" after it's constructor

Arduino 1.8.12
Teensyduino 1.51