No data exchange between ES and external tools is possible except via the SavedVariables.
There is no way to transfer the data of any ingame book etc. directly to an external audio processing tool.
Means: You always need to do a reloadui -> Write to the savedvariables to parse it with the audio tool -> Play the sound then.
Do you really want to do a reloadui each time? Even then you'd need an external tool, no addon, to read the sv file and send it to some text2speech engine (of windows e.g.).
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