Since you are using RCA cables for vid and sound, it's time to use something else to see if your audio cables are working and if what supplies that audio on RCA is outputting such.
Model: Bravia KDL-40W3000
I currently have no sound coming from my tv just picture, this problem started to happen today and I tried calling sony but they were clueless to help me out. I Checked the rca cables everything is in the current order and in tight. I never missed with the tv since I bought it from day one, which was about 3 or 4 years ago.
Current audio settings:
MTS: Stereo (greyed out)
Speakers: TV Speakers
Audio Out: Variable
I do not have a surround system.
Everything in sound mode is greyed out as well. I get a message when I try to select one of the sound setting such as treble stating "Available when sound is output from tv speakers."
I am stuck now and I hope I do not need to get a new tv now.