I had a SONY Bravia HDTV (KDL46EX500) that allowed you to switch the audio output between the built-in speakers and a fixed-level RCA-style line out and an optical out (no headset output on this one). The fixed-level line out and optical line out are options meant for A/V Receiver-based setups, which are controlled by their own remote control. The idea is the audio gets sent at the proper, known level and amplified later with the volume controlled by a separate system presumably with its own remote control. But I had a set of computer speakers - the Logitech Z323 2-way + subwoofer speaker system - that only had RCA inputs and a 3mm headphone jack input and only a volume control knob.
The first thing to consider, obviously, is if there was a new firwmare available for the TV that would offer this option. Unfortunately, there was not. It also turns out that when you download the firmware, you may just be downloading a patch or the full system, but it's hard to tell because the contents of Sony's .pkg files are encrypted. As I was researching more, I came across door #2.
It turns out that some of Sony Bravia TV's run Linux. I found very much information in the comments section at https://hackaday.com/2012/06/20/getting-root-on-a-sony-tv/ including a link to a walkthrough of using a buffer overflow hack to become root (really it's available at https://github.com/CFSworks/nimue) , which is then used to open a port for remote, telnet access as root that lasts until the TV is restarted. Sadly, this approach didn't work with my model - possibly because it's not a Smart TV. I also don't see my model listed on Sony's source code distribution page. This truly would have been awesome and I believe was my way "in" to really get at those menus and the commands they run.
OK, so much for sophistication. It was back to basics with this one. The basic idea is to open up the back cover and connect an extra set of wires to the speakers, which are volume-controlled by the standard TV remote control (perfect!). The extra wires would then feed my RCA inputs. That's great - the catches are 1) you have to isolate them to avoid a ground-loop and 2) you have to convert the signal level back down from speaker level (Volts) to line-out level (milliVolts). If it weren't for catch #1, I could have just used a cheesy voltage divider network with some spare resistors to drop the line level. Luckily, there are devices that handle both steps and which are typically used in cars to connect a subwoofer amp to the stereo system using the signals at the rear speakers. OK, it's a bit uglier because now I have extra wires coming out of the back of my TV - although theoretically I could have tried to mount some RCA connectors on the back of the TV, but that would have required modifications which were not "undoable," plus who looks behind the TV anyway? ... but it works. I just set the volume on the Logitech system about halfway up and control the input signal with my TV remote control. I additionally get the benefit of better treble coming from the TV's speakers - sort of like having a center speaker. Sadly, the total cost wasn't as good as the hacking methods, which would have been free. It ended up costing about $10 + some lose wire I had lying around. I ended up going with the PAC SNI-35.
Given my familiarity with receiving and sending IR remote control signals, I considered using the fixed line out and hooking a stepper motor up to the volume control knob! It would have been a truly over-the-top solution, but it would have been kind of neat.