The set volumn does not work.
Stopped around 2.4.x
Also, waits etc, needed to be "re thought"
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I am unable to control the volume in home minis as well. But I read recently that issuing the speak with the desired volume but the TTS string being null "" worked for someone.
I have had many tries at this over the years and the solution for me was to Speak a blank char, wait 5 seconds and then the set volume.
Speak wakes up the speaker to receive a setvolume command.
Here is a part of my system start piston that finally works and the volumes to a known value on a boot.