Hi, I am sorry you are having difficulties with the microphone and Google Assistant. There are a few things that can cause issues with the microphone on your phone or tablet. For example, an external device such as headphones may be preventing the microphone from working. Other times, you may hear a crackling noise, or sounds may be unclear in your recordings if the microphone is having problems. try this guide here: https://www.samsung.com/us/support/troubleshooting/TSG01001249/
Please let me know if this helps.