@Perth560 Yes, you can use the audio files and text generated from TTS or STT for commercial use. Please ensure you are using a speech resource that is billed for your subscription. The billing will be based on the pricing as mentioned in the pricing page. If you are using a free account or credit you should upgrade to Pay as you go and use the selected tier.
You need not seek permission or license or pay loyalty to MS for using the output files generated from your resource as long as you use your own content as input to the service. Please ensure to follow certain responsible guidelines listed here while using synthetic voices in your applications and ensure proper disclosures.
You can lookup more details about the compliance and privacy terms from here. I hope this helps.