Hey there heating_12promax,
Are you running into any errors or messages about the temperature? This guide can help with those: If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support It explains situations in which your iPhone might get warm through normal use. If you continually see temperature alerts and they are not resolved by following the guidance, you'll want to reach out to Apple Support to look into options: Contact - Official Apple Support
With regard to the sound issue, please remove your case and screen protector, if applicable. Restart and test without them. Create a voice memo and play it back. Do you still hear low sound or distorted audio? Does it happen with FaceTime calls or FaceTime audio calls? This information can help us narrow down the possible cause and pick the best steps forward.
Take care.