Hello @Melvin Vega
Thanks for reaching out to us, there are possible two reasons you can have a check:
1 - the quotas and limitations: there are quotas and limitations for Speech Service, below document mentioned all the limitations.
2 - the limitation of pricing tier - I will give the free tier limitation, please take a look:
For your question "Now, standard tier says that we have to pay $ 2.5 per hour. How is it work? Do we have to pay this all the time or this is by request by hour?"
The price you pay is regarding to your audio hour, you will pay $2.5 for every audio hour you input to Speech Service.
Hope this helps.
Regards,
Yutong
-Please kindly accept the answer if you feel helpful, thanks.