I just released version 1.0 of my ‘Make Spoken Audio of Text’ shortcut.

This shortcut creates an .m4a audio file from any text that you pass into it. By default, it’ll generate an audio clip with one of the default Siri voices, but you can change the voice to any one loaded on your device. It will ask you for a filename, and then will save the audio file to your Downloads folder.

Get it here from the HeyDingus Shortcuts Library.