@markuseisl,
thank you for sharing, I have recreated your workflow and I solved your problem, you can check solution here:
https://github.com/dinofilipovic/GoogleCloudSpeechtotextAPI
Best solution for CSV and EXCEL is to create a DataTable and then you can fill row that you want to fill specified in Add Data Row activity.
https://docs.uipath.com/activities/docs/add-data-row
In my project I have removed credentials for Google API so you need to put your JSON file in file path.
Cheers,
Dino