Do you mean that at first you received the APi with the 18.04.1 LTS Bionic,
then you installed the 19.04 Disco from the DLI image,
and then that you failed to reinstall the 18.04.1 Bionic from its DLI image?
Is that so?
Did you follow exactly the same procedure?
Would you describe your procedure in more details than what is on the READEME file?
At this moment, on my side, I received the APi with 18.04.1 LTS three weeks ago
and then last week I installed the Disco 19.04 whose pulseaudio drives perfectly the sound.
I have not tried to go backward with the downloadable 18.04 image.
Did you check the 18.04 zip file integrity before you extract?
I am not a xobyot,
but you are the one!!!