There's a little-known feature of iOS called "sound recognition" that constantly listens and pops up an alert when it recognizes certain sounds that need attention, such as a crying baby, doorbell, or siren.
There's of remote sign language interpretation and real time captioning services available. In the US, we have publicly funded video relay service (which translates between sign language and English for phone calls) and captioned calling (which provides captions for telephone calls). There are commercial equivalents of these services available for in person interactions and other countries.
I don't know of any apps, but 20 years ago at least there were state run services for this. You call a number and someone will translate between your voice call and the ttd that the deaf use. (20 years ago a friend of mine worked for such a service, we have both moved on but I assume the service still exists)
My understanding is that it's majority abuse -- people outside USA using it to call electronics stores to try to make fruadulent orders (to the point where stores refuse service to TDD users), and phone companies collecting huge fees for running the service.
Is there one for deaf people? i.e. converse over video and text