@尧 何 The free credit of 200$ expires after the first month but you can continue to use your speech resource if you have chosen the free tier. As long as the usage is under the free tier limits you will not be billed after upgrading your account from free to pay as you go account. Please refer here for more details.
When you sign up for a free Azure account, it comes with $200 in service credit that you can apply toward a paid Speech service subscription, valid for up to 30 days. Your Azure services are disabled when your credit runs out or expires at the end of the 30 days. To continue using Azure services, you must upgrade your account. For more information, see How to upgrade your Azure free account.
The Speech service has two service tiers: free(f0) and subscription(s0), which have different limitations and benefits. If you use the free, low-volume Speech service tier you can keep this free subscription even after your free trial or service credit expires. For more information, see Cognitive Services pricing - Speech service.
If an answer is helpful, please click on or upvote which might help other community members reading this thread.