Unfortunately, the Heyer mysteries are either not KU, or not WhisperSync, except for one where the Audible is >$7.
However,
Cocaine Blues, that first Phryne, is $.99 (though you already have it, I believe) and $2.99 for the WhisperSync Audible version. I can attest that the Audible versions are well done.
Also, consider watching the series. It's being
streamed on Acorn TV, which you can try for free, I believe. I'd be surprised if Essie Davis, playing Phryne, didn't captivate you.