Hi there,
We have 3 players (mpv, mpg123 and gstreamer) to use. SpeechSynthesizer and Alerts prefer mpg123 which is more responsive. AudioPlayer likes gstreamer > mpv > mpg123. Gstreamer supports more audio format and works well on raspberry pi. We can also specify the player of AudioPlayer using the environment variable PLAYER. So please try below commands to enable the voice. thanks.
sudo apt install mpg123
PLAYER=mpg123 python ns_kws_doa_alexa_with_light.py