Sudden strange problems with iPhone 12

Hello That_Username-Is_Taken,

Thanks for posting in the Apple Support Communities.

It sounds like you may be having a problem with the network connection and speaker of your iPhone 12, is that right?

If we understood you correctly, let's first check if you're using Wi-Fi Assist. If this is in use, your iPhone may be swapping to cellular data if the Wi-Fi connection is weak or slow at the time. Check out the following resource for more details on how this feature works, and we'll include steps to check this below: About Wi-Fi Assist - Apple Support

Turn Wi-Fi Assist on or off 

Wi-Fi Assist is on by default. If you don't want your iOS device to stay connected to the Internet when you have a poor Wi-Fi connection, you can disable Wi-Fi Assist. Go to Settings > Cellular or Settings > Mobile Data. Then scroll down and tap the slider for Wi-Fi Assist.

If adjusting Wi-Fi Assist doesn't clear up the connection issue, confirm if this occurs with your cellular or mobile data disabled. This will allow you to use Wi-Fi only to see if we can isolate what could be contributing to this. In addition to isolating this, be sure you have the latest app updates installed. If you need assistance updating apps, take a look at the following resource: How to manually update apps on your Apple device - Apple Support

As for the audio that you're not hearing, can you confirm if this happens with all sounds? We want to be sure we understand you correctly when you mentioned the audio doesn't play. If this is correct, we would suggest giving the steps of the following resource a try as this relates to sound related concerns with your iPhone: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

Let us know what you find, and provide any additional details for us to have a clearer understanding.

Have a great day!