Q: The speaker on my iPhone seems to be blown out and sounds muffled and very low quality all the sudden.

Try the troubleshooting help here >  If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

If nothing there helps, check your warranty coverage >  https://checkcoverage.apple.com

The iPhone may need repairing >  iPhone Repair - Official Apple Support

Hi,

There are definitely some steps you can take on your own. See If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support for steps to try. If it doesn't work, you get get service in different ways depending on the country you are in. Typically, you would have to go to your carrier, an Apple Store or an Apple Authorized Service Provider, or call AppleCare so they can send you a box to send them your iPhone.

You can go to https://getsupport.apple.com/ and select your countries to see how you can get help from Apple.