But if it was mine I'd start with a factory reset and if it has a headphone jack try that port as well.

I might consider testing with a sound bar or headphones.
https://esupport.sony.com/US/p/model-home.pl?mdl=KDS55A2000&LOC=3 notes how to connect headphones.

There are also more ideas under "Sound & Audio" at link.

Many options here.