Hello HMSJ1!
If you have a case or screen protector on your iPhone, a great isolation step to try is removing them to see if the volume increases. If the issue persists, restart your iPhone. Although simple, this step can help correct some unexpected behaviors. It's also a good idea to make sure your iPhone is running the latest iOS version since this can help with overall performance: Apple security updates.
If you need more help, reach out to Apple Support directly so they can look into this with you further. They can be contacted here: Get Support, or by calling the most applicable phone number from this list: Contact Apple for support and service.
Also try this link:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support
I hope this is helpful. Good Luck!