Is "varUt2" base64 format? Try something like below first:
Substitute(base64,"""","")
For Audio control, you can only access audio files from Media and online resource.
For your scenario, your audio file are dynamic converted, Media files are not working.
You could try to upload the audio files to online service which can access directly, and then passing the url in the Audio.Media.
If you did not want to invoke online service, you could try save the audio files to list attachment, and then access it by something like below:

Best Regards,
Meekou
YouTube
PowerApps
PowerAutomate